first off, have to be 18.
I would recommend...whatever the hell you like. Air Force, you get to fly state-of-the-art jets, Navy, you get to blow stuff up with missiles and sail the seas, and then blow more stuff up, Army are the grunts of the ground troops, and Marines are the hard-asses.
Problem is, you might get burned to death, blown the forget up, or you could drown, get shot and die a horrible slow death, or mutilated.