@Kyra: When you are in a fight, click on the white sign beneath your character which shows which weapon he/she is using. You get a menu from which you can choose the character's next action. To choose a spell click on the book and you get a full menu of the character's spells. Choose a spell, its intensity and, if necessary, a target. The character will then say "Okay" or whatever and cast the spell when it's his/her turn.
Some spells may be cast when you are not fighting. Such as "Light", "Armor" etc. To do this activate the character and click the book. Then do as described above.
Hope this helps.
