You're doing it right. Just keep playing, and you'll learn as you go. Don't be afraid to search for answers online, or compare D&D to BG3. Later, when you understand the system and a spell's impact to your journey, you can talk to Withers and respec your character, changing your spells and abilities. You can also just use an online guide. Many of the guides have optimized builds that work really well. That's what I did for my companions, since it was a bit much to learn every single class in one playthrough.

Edit: example, this Astarion build is off the hook amazing!