What nationailty are you as different countries have different criteria for entry. Bbut for all armies you have to be very fit. A good training plan would be the navy seals one:
Warm Up
Jumping Jacks
Loosen up [link to the workout]
Windmills
Trunk side stretch
Pull Session [30 sec break between sets]
Close grip pull ups
Wide grip pull ups
Close grip pull ups
Behind neck pull ups
Regular pull ups
Close grip pull ups
Push Session [touch chest to ground if you can] [1 min rest between sets]
dips [Shake and loosen up between sets]
dips
Regular push ups
tricep push ups
regular push ups
tricep push ups
dive bomber press ups
Abdominal [try to not take breaks for maximum muscle building]
Sit ups [do not yank head on sit ups]
Cross arm sit ups
Flutter kicks
half sit ups
cruncher sit ups
lizard stretch
atomic sit ups
good morning darlings
neck rotations
neck lifts
Legs
lunges
squats
charlie chaplins
squats
Cool Down
the warm up but slower and followed by stretchs for all major muscle groups
That would take care of the muscle building but you would need to either eat some lean meat or a protein shake within an hour of doing the workout, do it after school as it can be quite tough too.
For your aerobic system I would do some running. Mix it up with both sprint type stuff and some distance like 5km. What I did was I found a 2.5km lap around my house and would run that as fast as possible. If you can do this sub 12:00 minutes then I think you could pass the Uk army running test.
This could be a weekly plan:
Monday: navy seals workout
Tuesday: rest
Wednesday: 2.5km run, record your time
Thursday: 5km run @ your own pace
Friday: rest
Saturday: navy seals workout
Sunday: rest
Good Luck and good on you for wanting to join the military!