What grade are you in?
Personally its up to you. I never did JROTC and I'm doing fine here. I played football and baseball at McQueen, so naturally I was hitting the weights and doing sports.
If you are interested in JROTC, by all means do it, just from my experience it doesn't help much when you are here.
And just keep in shape in general. Go running, lift weights, eat right, exercise! You need to be in good shape if you want to go here.