If its good usable fitness your after i would recommend compound exercises which are a great way to get all round strength!
so things like deadlift, squats, bench press, military press, pull ups and push ups
i assume because its for the army your after a good bit of endurance! so try not to go too heavy on the weights so your able to do sets of 15 - 20 reps ... the more reps you do the more endurance training and less strength training it is.. so find a balance that suits you! i personally aim for 8-10 reps, but im after strength more then endurance
and as others have recommended... swimming, running and a good diet will give you heaps of energy!
as for strength.. i like to use the general rule that every man should be able to cope with his own body weight! so if your 12.5 stone like me... should be able to do a 80kg bench press (including the bar) or do 10 pull ups no sweat!
Anyway.. good luck on your training!!