Basic do's and do not's when starting:
DO -
Decide what fraction you want to be apart of.
Take a look around the map.
Start sinking smuggerlers or low level bandits/pirates.

DO NOT'S -
Sink another nations ship, yes these means the Brotherhood of the coast aswell.
Squander all your money.
Waste time.

So, hopefully you have not gone mad and attacked another nation's ships and chosen a nice, easy smuggler or pirate ship.

What to do once the battle commences,
1. Aim your ship directly at the hostiles, yes this can be seen as a stupid move but do it anyway.
2. Open fire the second you get the chance, if the hostile outnumbers you, do NOT attempt a boarding action.
3. If you outnumber the hostile, you may attempt a boarding action, be warned that a boarding action is only recommended if you have superior troops and/or numbers.
4. Ah, now you have killed the hostile crew, or sunk it. I would reccomend scuttling the ships to start off with as a glitch allowes random loot to spawn that was not on the ship when you looted it before, an example is, I attacked a Large Flyut, they had 10 suger cane and 15 metal, I scuttled the ship only to get a lot more, valuable goods.

WARNINGS:
A boarding action is a very risky move, there have been cases where I have outnumbered the enemy 2 to 1 but I end up getting captured due to them having either "superior units", or just outright bad AI path finding.

Dispite what I said in the previous warning, the AI naval combat will attempt a boarding action if it feels that it can win, you can tell this by seeing if the ship turns to give you a boardside just before you ram/board it.