Check the sticky. It depends on what branch you want to go in. There's the
Naval Academy at Annapolis , the
Military (Army) Academy at West Point, and the
Air Force Academy in Colorado.
Should you receive an appointment, there's no tuition. I don't know much about the other academies, but at West Point, you basically pay off your education by serving in the Army on Active Duty for 5 years, Reserve for 3. If however, you make it past the first two years and drop out after making your "cow commitment," you'll owe the government 250,000 USD.
There is however, a deposit made to cover your uniforms and whatnot, at West Point, it's 2500 USD I believe, but other than that you actually receive a salary.
*IIRC*
I guess you can figure out where I plan on heading after high school
. If you have any questions about West Point in particular, I'm pretty knowledgeable about it considering I'm not a cadet there. Otherwise, I've got a buttload of books about the USMA in my room I could check with.