Sport Help, three weeks to get fit

[callum ban]

Right, im 16 and am leaving highscool; this year. Im hoping to join the british army and in three weeks I a going to a brief. There I will be told about all the jobs available, take an interview but also take a medical, this includes fitness and is followed by a mile and a half run which has to be done in under 12 minutes. Okay, I really need to become fitter, not just for the run but for joining trhe army in general, so please I need to know the best foods to eat and also I need to know what exercise I shopuld start doing. Any help would be excellent