If you were accepted to a school they usually give you a sheet that asks whether or not you are going. Just mail that back.
If you haven't yet been accepted:
1) If you finished your interview you can email or call the school and tell them you would like to withdraw your app. Or you can wait for them to get back to you.
2) If you have an interview scheduled you should call up the school and tell them to cancel your interview and withdraw your app. That way someone else can grab the spot.
3) If you haven't been offered an interview yet you can either wait for one if you are still interested in the school or you could call them up and tell them you would like to withdraw your app.