In order to attack an enemy creature you must move into it. Success depends on the attackers combat rating compared to the defenders defence rating with a random factor involved. Move the cursor to the target and press S to fire. The target must be in line of sight. Select spell Obvious really.
Neutral spells have no effect either way.
The lighter the colour the name of the spell is, the higher the chance of it working. The overall state of the world is indicated by the number of symbols on the main option menu. Real creatures have a chance of failing, but are unaffected by the disbelieve spell. Other than that the two work the same. If you are player one then you get to cast your spells and move first.