The Steam client they have made is essentially the vanilla client with less jank and more polish. Let's just hope they bring some QoL improvements that the 3rd party clients can provide, without them I fear the steam client will dwindle in usage. That being said, this client appears to be a much more enjoyable and less frustrating version of the original client, it is perfectly serviceable for the average Joe.

Some tips for you;

Join a Clan Chat or CC. I recommend any decent YouTube CC such as 'UIM Verf' (the one I lurk in) or 'J1mmy'.
https://oldschool.runescape.wiki/w/Clan_Chat

If you need help with quests there is Slayermusiq1 on YouTube who provides guides
https://www.youtube.com/user/slayermusiq1

If you need help with skilling methods there is Gnomonkey RS on YouTube who provides guides
https://www.youtube.com/channel/UC6lIpKC3lNPxeXryY-wH9JQ

The best source of information on anything Runescape is the community wiki, there is a button in-game under the map that says 'wiki'.

Here's a direct link to the wiki's page on using the in-game wiki tool
https://oldschool.runescape.wiki/w/Wiki_lookup

Here is a link to the wiki's homepage
https://oldschool.runescape.wiki/

If you do decide to get a 3rd party client ensure it is rule compliant and download it directly from the correct URL, remember you use 3rd party clients at your own risk.

GL, don't get scammed and have fun losing the remainder of your free time to the endless void of despair that is OSRS.