If your application is incomplete (i.e. secondary hasn't been sent in), don't bother withdrawing. You will fall into the "incomplete" category without doing anything.
Most applicants withdraw only post-interview, if they've been accepted elsewhere. At that time, just send a friendly letter to the Dean of Admissions.
It is also appropriate to withdraw when you are offered an interview if you've been accepted to another school. If you're invited to interview, it's your obligation to RSVP. It's best to do this by phone, so the school can contact another applicant to fill your spot on the interview day.
Remember that you'll be jumping through the hoops of the application game in another 4 years when it's time to get a residency. Just be polite and don't burn bridges.
Cheers,
doepug