You could look up the DnD 5e rules for Sorcerer, it's made the same way.
Sorcerers have known spells which are all the spells they can cast, and they can cast them with spell slots. It takes a level 2 spell slot to cast a level 2 spell, but a level 1 spell can be cast with a level 1 or level 2 spell slot.
Sorcerers don't prepare spells, they just know them, but they can swap one out at level up, plus they learn more with level ups.
Sorcery points are used for metamagic, which allows you to modify spells depending on what the metamagic is.
I haven't played sorcerer in BG3 do I'm not sure if they add or change anything from base 5e, but checking the 5e info should be a start.