Well, first, you have the event of
A Unit Starts the Effect of an Ability
then, the condition of
Ability Being Cast Equal to (your ability)
Then, in your actions, do whatever you want the spell to do (you'll have to learn atleast GUI if you dont know it)
Useful functions: Triggering Unit = the unit that casted the spell, and is more efficient than Casting Unit. Target Unit of Ability Being Cast = what unit a unit-target spell targeted. Target Point of Ability Being Cast = what point a point-target spell targeted.