Best class for a first baldur's gate 3 run if you've never played d&d?

If the class choice still worries you, play on Explorer for the first run. It's the easy mode and it lets you learn the systems without wiping every second fight, you can bump it up later.

Paladin. Hits hard, survives mistakes, and the smite is easy to get: you land a hit and choose whether to spend a spell slot to make it hurt more. Fighter is even simpler but a bit flat. I'd skip Wizard first time. It's very strong, but you're juggling a long spell list and it dies fast if you stand in the wrong place. And don't stress about building wrong, Withers in camp will respec you for a bit of gold.