Just adding my voice to those saying that while it's probably good to have a basic grasp of D&D (and 5e) it's also likely best not to overplan given how much we don't know. Of course, we should all play however we please, but personally I'm intending to largely wing it on my first playthrough and see what I see, and then will probably try some more planned out playthroughs after that.
I also like playing magical rogue type characters, and I think they're also a great choice for a first playthrough as it makes a lot of sense that they're curious and experimental, so you can get to see and play around with lots of aspects of the game as a player while still leaning into the roleplay aspect that I personally think is one of BG3's strengths.
