Just like in 5E D&D there are limitations on what spellcasters can do in Baldur’s Gate 3. Spellcasting carries some of the complexity of the TTRPG, but with some of the minutia stripped away. The Schools of magic are there for example but don’t have much impact on gameplay, they’re just there to classify magic types. There are no special rules for each Magic School. As more classes are added closer to release, expect this to get more complicated.
As for gameplay impacts, there are spell slots. The spell slots will differ by class choice, and some abilities will also have their own spell slots of a sort. Some spellcaster types are restricted by what spells they have prepared. This applies to classes like Wizards and Clerics can prepare spells during a long rest to use for that day of adventuring.
To prepare spells in Baldur’s Gate 3 you need to be out of combat and have your party standing by. You can prepare spells anywhere, you don’t have to be back at camp. Once you are out of combat, open up your spellbook for the character you wish to prepare. This UI contains all of the information for the party in terms of actions, cantrips, and various spells. You can see which spells are prepared for a spellcaster by looking at the list and looking for the highlighted elements. Any greyed-out spells are not prepared for that day and cannot be used right now.
You can only prepare spells that you have an open spell slot for. To change to a different spell in that slot, simply click the highlighted spell and then click on the spell icon you want to use in its place. The trick here is that both spells need to be of the same level to be swapped. You can’t swap in a level 3 spell for a level 1 slot.
The spells you have prepared should also show up in your hotbar once prepared. That’s the clue that you’ve done this all right.