I know the navy and USMC will send you to get some civilian training (on their dime) before they accept you for flight training. This is done for two reasons: 1) Give you some minimal flight background and 2) Perform a quick evaluation on your learning potential (academic grades don't give a great indicator of your ability to fly airplanes).

The key to getting a military slot is good grades, good test scores, and being well-rounded (sports, leadership, outside activities). If you want to go that route, try to get an ROTC or academy scholarship if you're still in HS or early college.

Note: Getting into military flight training is one thing...getting into fighters is another. The top performing trainees often get their pick of aircraft, but there is also an element of luck. If they really need helicopter pilots that month, then EVERYBODY goes to helicopters. Navy/usmc you are stuck for life, AF you might be able to switch later, but probably not.

If you REALLY want fighters, try to join an ANG or AF Reseve fighter squadron...that's the only way to be sure.