Follow the instructions in the spell's text. If the enemy has failed their dispel attempt (or not even attempted one!), the spell is cast successfully and its effect is now resolved. Each spell in Warhammer provides all the information you need. Many spells inflict hits or wounds on your enemies – you can find out how to resolve these in the Shooting chapter.
Remember that if your spell was miscast, you'll need to go back and resolve the result of the miscast as soon as you've finished resolving your spell effect.
Note that if a spell is cast but cannot reach the target for any reason, the spell still counts as being cast, but won't have any effect.