half of all military training is the actual mind flush. your way of thinking becomes different after the fact. just working out wont make you like a ranger.
Start by joining your H.S. football team...most H.S. programs these days have near year round strength and conditioning programs...if you go thru a complete 4 year program and have any athletic ability at all, you'll be quite the physical specimen by the time you graduate. Playing H.S. football and dealing with the grueling workouts and the coaches yelling at you is about as close as civilian teens can get to top level military training. Also consider cross country.
參考: a U.S. Marine vet who spent 11 months in Iraq in 2004-2005 (3rd Battalion, 1st Marines, attached to 31st MEU), mostly in anbar province. I am now a multi degreed engineer working for America's largest defense contractor. I played H.S. football (RB and CB at different times) and also ran a partial cross country schedule. When I got to Parris island I was one of the most athletic recruits in my platoon and found the physical part challenging but not anything to really fret over.