Well for which first playthrough?
For my solo good playthrough I’m going half elf lore bard.
For friend A playthrough I’m going high elf storm sorcerer.
For friend B I’m going half orc battle master fighter.
For friends C and D I’m going our original DND characters so half elf rogue
Side quests in Diablo and bg3 are vaaaaastly different. They’re much more like Skyrim but that still hardly does it justice. In Diablo you find a burning building then the survivor says you need to find her sister in a different unrelated dungeon and you get a nice quest marker directing you there. In bg3 you find a burning building and if you make a skill check you might save the last survivor, then you can extort them for money or just kill them if you really want, then they mention their sister is trapped, you can ignore it or promise help or promise more of a reward, then you get a journal entry describing where you’re currently at in that quest and it’s up to you how you handle it from there.
You can try reading through the 5e players handbook or googling whatever you aren’t understanding followed by 5e. For example “spellcasting 5e” will explain spell slots, ritual spells, learning new spells, and preparing spells in the easiest way they can. 5e means 5th edition and is the version of DND the game is based on.
Some other quick tips, most classes know a lot of spells but can’t prepare them all, you can change your prepared spells at any time outside combat. A ritual spell can be cast for free without using a spell slot outside of combat.