How, exactly, do you write that letter? Easy - use this template!
Dear Ms. Name (or Admissions Office):
I am writing in response to your offer of admission to the (Name of) University. I appreciate your interest in me, but I regret to inform you that I will not be accepting your offer of admission. Thank you for your time and consideration.
Read each colleges instructions and follow the directions carefully. These colleges need to know your answer now, so they can make their plans, and provide scholarships and admission offers to others. Don't wait until May 1st if you know in advance you won't go to a particular college and can reply earlier. Answer them ASAP when your decision is made, but absolutely not after the deadline.
It's difficult for students, and parents, to make that final decision to say no to college offers, but I encourage you to move forward in faith. God can lead you when you are MOVING forward, but if you are stagnant, you're more like a donkey than a faithful follower. In our family we call this "point and pray" - you look ahead, point in the general direction, and pray. God will guide your steps.