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ALL YOU NEED TO START A GOOD GAME
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Rev.2021-09-03 : Added some default settings that to me is better to change
Rev.2021-08-26 : Added a paragraph about GameScripts
Rev.2021-08-25 : Final)
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Preamble:
This game is about money (in competitive mode),
(I guess, that also a coop mode could exists between friends or on some servers, why not)
or mind challenge, shaping the cities as you like (relaxed sandbox like a sort of city building).
But what I prefer is the challenge of creating good trasports design, logistic.

PRELIMINARY OPERATIONS
First of all, let's prepare the game:

Font size
If you see the fonts too small, let's make them more readable, so,
- EXIT THE GAME
- search config file "openttd.cfg", usually in $user/documents/openttd
No matter if you use native OS file browser and its search or a powerful MultiCommander ,
I suggest this from command prompt that is very simple and fast:

You can keep original values in the cfg file, just ensure the latest are above the others.
Program use 1st value for each variable.
play with (for example) :

small_font = Arial Unicode MS Bold
medium_font = Arial Unicode MS Bold
large_font = Arial Unicode MS Bold
mono_font = Consolas
small_size = 12
medium_size = 13
large_size = 14
mono_size = 12
small_aa = false
medium_aa = false
large_aa = false
mono_aa = false

- START THE GAME

Open "Online Content"

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Select a graphic set like aBase (double click) - it's personal choice, there are others

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Select the scenario tutorial (double click)

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Let's include also BigGUI, a nice button bar set

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Clear the filter field

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Download all the selected items

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Go here

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Managing NewGRF howto[wiki.openttd.org]
Move BigGUI from the bottom cache list to the top active list
In the upper frame you should see only the one in the white rectangle:

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To be frank still not clear to me what this button is exactly for.

Changing of NewGRFs work anyway, I guess that could have to do mostly with changing the parameters of the used NewGRFs.

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So change parameter of BigGUI and then APPLY CHANGES

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Change the graphic set in the main settings

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Finally play the tutorial

Settings (enable view mode to see ALL settings)
NOTE: some are global, some affect only new games, so check tip at bottom to know which.
To ensure the below are always set "from now on", configure them before start a new map
(means: if you have already maps in progress you have to change these settings again).
- Pause: do all (you probably will sometimes stop game and build in relax)
- Rail 90°: NO (c'mon, be fair)
- Cities build roads: NO (you manually have to add and fix broken roads firstly),
and this way you have better control on the city growth.
- Duration of error messages: (increase if 2 seconds is too low)

Other Settings that can be changed before start a new map:


More AddOn (newGRF, actually):
- Manual Industries II :
Industries (complex topic[wiki.openttd.org] for now) can give some problems:
In regular game, they may change production type, or in some conditions they may announce closure and disappear, leaving your newly designed track in front of an empty brown rectangle.
While this kind of "mechanic" is part of the game (considered annoying by many players, though), you will probably want to play a more stable map, only allowing for the creation of new industries.
This NewGRF help you to avoid these nasty surprises.


- SuperSimple AI, Simple AI (+settings), AIAI, many others (CPU competitors)
*but* begin playing sandbox, alone, to become familiar with gameplay.


NewGRF:
- Make sure you have read the WIKI NewGRF[wiki.openttd.org] as already suggested before.
- General rule: the AddOns officially known as "newGRF", have to be added (and configured the settings if available) before start a new map. If you want add a newGRF in a running scenario (means: map already in progress), it is possible to force a newGRF to a current game enabling the "Developer mode", however there is very high risk of game corruption and crash, therefore, not recommended at all. Ensure to enable the list of newGRF you intend to use, and their settings, before start a new map.
- To remove the newGRFs that you don't need, disable them from the active list in the newGRF interface, exit the game, with your file manager navigate into the "content_download" folder and delete those you don't need anymore. With care.
- Very useful: you can create preset (groups) of NewGRFs

Anyhow, before start a new map, double check parameters of the NewGRFs, to ensure they have not changed for some reasons. Preset Groups, NewGRFs and parameters are saved in the file openttd.cfg, but moving from cache to active list they are added in "reset" status. So, be warned.

InGame:
- Practice with flat map 128x128, low/normal number of cities and normal number of industries
- Start playing temperate maps that don't have special needs
- Generate map until cities are equally distributed on the map
- If you want to try trams[wiki.openttd.org] in your cities, take care to enable a set before start the map. They are very nice and tracks can be built on grass. Note that the suggested (in the wiki) "Generic Tramset 0.4" is low res, may be you want try the high res "CZTR Tram Set" 32bpp.
- High Res: in general, you can search all the NewGRF with high res., writing "32b" in the filter.
- Playing sandbox ignore subsidies proposal, initially. Instead...
- Ensure there are some coal mines, and bring to power plant(s)[upload.wikimedia.org]. Easy revenue$ to play with. After coal, try the other production chains to get more money.
- Place rail as usual, keep CTRL pressed to remove. There are other combinations[wiki.openttd.org]
- Use only modern "path signal"[dpk.io] and in settings, configure to swap those only. Once the direction of the train is assigned, use the blocking signal for a more logical and aesthetic aspect. However, initially use only path signals, These signals do everything you need for the trains.
- Transparency - It is in the button bar of course, but CTRL+x is faster. Useful when you build.
- Not enough? https://wiki.openttd.org/en/Manual/Hotkeys
- Evaluate situation of your routes using button Maps -> (choose) Cargo Flow

- And your means of transport too (icons train/bus/etc), sorting by profit (previous year),
and pay attention to those in red, or with low earning, even though you have to consider the global picture of your transports because may be that for a period some are in red but required as support.

- Stations list, check for raw/semifinished/goods/etc. in stby to be moved to improve economy.
Sort in order to have at top the overloaded ones. Ah yes :) Passengers and Mail too, of course.

- If cities grow slow, you'll have to help the smaller ones with Local Autority -> "Funds new Buildings", but you should have so much money to don't know what to do with it.
- If Local Authority does not allow you to build, place trees in the city area, several times.
- Learn how replace[wiki.openttd.org] single vehicles without loose orders or an entire group
- When electric engines arrive, remember to convert all your tracks, drawing a grid area
- Later you'll have to deal with more rail types. Be prepared[wiki.openttd.org].

A few words about MonoRail and Maglev:
In regular game, when transition from electric to monorail/maglev happen,
there are several strategies to replace vehicles and rail types to upgrade the network.
Although "old" vehicles can run on electric rail, with monorail and maglev is more complicated
because there is no compatibility. Up to you G to choose and try your best way to upgrade.
But, and this is very useful to know in advance before begin the map of your life :) ,
you could read about these newGRF[wiki.openttd.org]:
- No Monorail and/or No Maglev : a bit extreme but not all like to manage superfast trains.
- Progressive Rail Set : allow vehicles to run on previous rail generation. Transition is softer and not much different from the change from old to electric, except that you need new depots type.
- Universal Rail Type : it might be too permissive and simplified in some aspects. Proper use, afaik, is: send all train to depots, convert rail network to universal rail, place new depots type close to old ones, start replace vehicles usable with monorail(year 2000)/maglev(year 2020), convert your network rail back to monorail/maglev and you are done. For both read the notes in the newGRF page.

Autosaves and Saves
- Settings : autosave interval
- openttd.cfg : max_num_autosaves (if you want many montly autosaves)
- Recommendation of course is to save often choosing appropriate filename, eg: "Before station"
- You can then go with your favorite file manager in the saves folder, ordering by date desc.
and delete all the unneeded, not one by one with the game interface.

How cities grow:
I've read many many many forum / discussions.
There is a lot of confusion and misunderstanding about this argument.
So let's clarify what is really needed.
- https://wiki.openttd.org/en/Manual/Towns
there are lot of important information, but read and then double read these sections:
# General Conditions for Town Growth
# Tips for Town Growth.
Basically, no need to get crazy with goods except to ensure money entry,
because if you set 5 bus station in the city, you have created the growth.
If you set 150 bus station more, growth does not change.
This cover also the question:
what if a station is away from the center and named "CityName" + "SomeSuffix"?
No problem, important is the local authority to which the area refer to.
Use button "?" to inspect tiles/elements on the map
Look this video (link below). City can grow even not connected to anything outside,
and even if the center of the city has not an its whatever station/stop (personally tested).
Master Hellish points out very well this concept How To Grow Towns and Cities
also explained by him with full details: Towns and Cities
Obviously,
if you try to search tips&tricks for this game, there are mega-tons, but for this specific case,
you could learn some interesting workarounds, for example:
Place a bus station near the center of the city (around its name in the map), this bring slightly outside the influence of the center city name, so you can place a rail station using CTRL, outside the center, but in the influence of the bus station that is extending the catchment/influence area and allow to have same name to both bus and rail station (not east/west/south/north), otherwise not possible. They act like an unique station. Note that, after placing the rail station you could also decide to remove the bus station with the related icon busStation->buldozer, not with the demolish icon that would destroy both.
BTW, this has to do with setting "maximum station spread", but better ignore for now.
Later try to check coverage/influence area of an airport and what could be possible with that.

Finally, Settings that evolve significantly your gameplay to a deeper and elevated level:
https://wiki.openttd.org/en/Manual/Passenger%20and%20cargo%20distribution
(Cargo) Distribution

To ensure the below are always set "from now on", configure them before start a new map
(means: if you have already maps in progress you have to change these settings again).
- Passengers + Mail : Symmetric
- Armored : Symmetric (check tip at bottom!)
- Rest: Manual
- Now add bus/cargo stops to a rail station (using CTRL, but no need if adiacent tiles)
and new (named) bus/cargo stop in the city, and create routes.
- Short explanation of symmetrical operation[www.tt-forums.net]
- Check in your stations the effects of this new toy, you will realize that routes have to be planned with more attention based on both passengers and mail destinations.

"I want more variety and complexity !"
Sure: after "Temperate" maps, you could try Sub-Arctic and Sub-Tropical[wiki.openttd.org] maps that have some more cargos and requirements for cities growth, then, sooner or later you will read about different industrial chains and some weird words like ITI, PBI (Pikka), WRBI[github.com], ECS, FIRS[grf.farm] and many more, which mean more industries, more cargo, more (much more) challenging gameplay. BTW, "more cargo" means that the default wagons set is not able to transport all the new cargos provided by these extension so you have to take care to add some vehicles set to comply the used "NewGRF Industries" requirements that you choose. Note also that some "NewGRF Industries" or additional vehicle sets may exclude monorail or maglev or both. Suggestion is to start some new games in 1901, 1951, 2001, 2021, check what types of rail are available, place a depot and see how the "NewGrf" set you have built, behaves. As usual "NewGRF List"[wiki.openttd.org] is your friend: check the "Industries and Landscape" section. This is a good place to start. And then search guides, tutorials, dicussions with G.
Playing these Industries "plugins", you will realize that wagons can carry different materials.
Up to you specialize them with "Refit" function.


The complicated but very interesting part.
Warning: these scripts completely change the way cities grow.
At some point you will realize that it looks like you are playing two games
- city building (passengers and mail)
- transports planning (industries)
They seem a little independent each other, except for a few extra requirements that you find in tropical and sub-arctic maps. If you are so brave, you will be able to join the two pieces further and have a more complete game. It means that the growth of cities will not only be based on standard cargos and roads + buildings, but could be improved by adding another level of difficulty to the growth: procurement of raw materials / semi-finished / products created by the different industries chains (described in the previous paragraph - but also play without should be possible) in your map, that will be required by your cities.
It is not easy to explain it in a few words. You should read some topics related to these scripts in the openTTD forum and get an idea. I am testing a few that I have selected by checking the ones that seem still actively maintained. My personal test list includes the following GS (GameScript)
- Renewed Village Growth[www.tt-forums.net]
- Renewed World Disorder[www.tt-forums.net]
- CityController v7[www.tt-forums.net]
- Simple City Builder[www.tt-forums.net]
Good luck.
Update 20210831: Still tests in progress, but so far, a very good pairing seems FIRS+RVG
(any comment from experiencede players is welcome, of course)

• Base game (aka "vanilla")
  
• Base game + Cargo Distribution
  
• Base game + Cargo Distribution + (Industries Chains *OR* Cities Growth gamescript)
  
• Base game + Cargo Distribution + Industries Chains + Cities Growth gamescript

Some official cheat if you are in trouble:
- https://wiki.openttd.org/en/Manual/Cheats
Often I use "magic buldozer" to destroy some broken roads that temporarily the "Local Authority" does not permit me to do normally. This way I don't forget to fix them later when I can.
It is also very useful to place some depots, change the date in the future, and check which "new vehicles" (buses, trucks, trains, trams, boats) exactly become available, in different years.

Congrats, now you master 5% of the game, but 100% of fun,
included all the errors you will do in design. Lot of lessons to learn :)

More:
- https://wiki.openttd.org/en/
- https://www.tt-forums.net/index.php
- MasterHellish's Video Tutorial (with index here on steam)
- G, but depending on what you are searching, pay attention that info are not obsolete.

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Thank you for taking time to read this guide, I hope it was useful.
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