Well, where are you in life? Are you a high school senior still trying to get a scholarship? or maybe you're asking with the intent to join ROTC as a non-scholarship cadet? I'll assume/guess it's the latter because then the question makes more sense.
I'd suggest showing up on day 1 at your ROTC orientation in the best physical condition you've ever been in your entire life. Start running and doing sit-ups and push-ups every chance you get. Show up clean shaven with a proper haircut and no earrings (believe it or not, my son's unit had 3-4 show up wearing studs on the 1st day of freshman orientation). Then, work your tail off in class and volunteer for all the extra-curricular activities and/or events you can without effecting your GPA.