Welcome!
Certainly I found researching D&D 5e incredibly useful in getting my head around the game. While there's a lot you'll find familiar, there seem to be some very different concepts that I didn't recognise from my old BG1/2 & NWN days, such as actions, bonus actions and reactions. And while a lot of the spells have the same name, the precise ways they work are often rather different.
Hopefully the full release of the game will do more in the way of teaching newbies how to play than EA, but some advance familiarisation with 5e couldn't hurt, as long as you then don't expect to see it exactly replicated in the game. Larian have definitely homebrewed a lot of stuff (and we're getting more info about changes now that may or may not align with changes planned to the D&D ruleset by Wizards of the Coast).
In terms of 5e websites, I've found
the 5e wikidot and
DND Beyond useful, plus there are sites like
Black Citadel which go beyond the rules as written and can help clear up uncertainties. And absolutely tons of other resources online!
But most of all, I'd say don't worry about it. After some initial bewilderment, I found the D&D rules implemented in BG3 reasonably straightforward to get started with, and though the tooltips we had in EA weren't perfect and were sometimes buggy, I expect they and the combat log are going to be really useful helping us understand what is going on.
I suspect it is going to take me a long, long time to master the ruleset properly, but it wasn't too hard to get started. And from that point, I could then just continue getting into more and more detail and dipping into web resources as and when I decided I wanted to understand a bit more about a specific aspect.
I hope you have a blast with the game when you play!
EDIT: And Blackheifer has just reminded me about
WolfheartFPS who has been really active producing YouTube vids on BG3 over the course of early access and I think generally explains things really well. There's a risk looking at any of his early access vids that much will be different in the full release, but he did say in one of his more recent streams that he was planning to do some guides when the game releases.