If you have experience with RPGs that’s all you need, the game will walk you thru it as others have said, I’ve never played d&d and having a good time

It’s very user friendly in terms of tooltips, and if you don’t make deliberately bad choices during level up (e.g. taking a feat that gives you a cantrip from the Wizard class… that scales off your INT score… while playing a Barbarian with 8 intelligence that can’t cast spells while raging) it’s fairly difficult to make an unplayably bad character.

There’s a few cases where some general knowledge of D&D is helpful, such as knowing to never take True Strike because it’s literally worse than just attacking twice and having some knowledge of good builds is useful, since it helps guide what you take when you level up. That said, there’s also entire categories of actions in BG3 that don’t really have an equivalent rule in TTRPG 5e, such as weapon proficiency attacks, so online cookie cutter builds don’t capture the full extent of what you can do.

Paladin and smite, all you need

In my opinion, It helps to know the rock bottom basics (like why is this d20 being rolled all the time and what the stats are) but it’s certainly not a necessity.

None really. The game will walk you through everything through tool tips and help menus.