You get a certain number of spells to use of each level per-rest. This can be increased by taking perks. For example, after rest you can use 3 level 1, 3 level 2, 2 level 3 etc until you rest again.
Class abilities can be either per-rest or per-encounter. It is best to use your per-encounter spells/abilities for every single fight because, well, why not as the reset after each fight.
For general combat, the best advice I can give is to sort out your auto-pause options. Also, pause often on your own to give orders. There is pretty bad AI pathing in this(unbelievably bad!), so you really need to keep on top of things.
Specifically for mages, you get a lot of control spells. The oil slick one is especially useful as I believe it is level 1?...Most classes do have good control spells though. Another really great feature of the game though is the engagement system which means you tanks can engage many enemies at a time and keep them occupied. Look for talents that increase engagement as well.
For skills, the one that matters most is mechanics. Make sure you have one person in your team with high mechanics for traps and locks etc. Lore is also useful as it lets anyone use high-level scrolls. Others are pretty pointless tbh.
Another handy tip is that you can at any time hire additional party members from any inn, or your keep. This means you can build any party you want(though they don't have any quests or personality obviously). It is best done early so you can shape them as you level up. Also a handy tip I posted earlier is that you start the game with a ring called Guarm's pledge that you can sell for 1500, which allows you to start the game basically with a full party!
The game can be overwhelming at first, but stick with it and you'll find it is deceptively simple