I would suggest that you start as soon as possible! Start running daily, try to get up to like 3 or 4 miles. I would do crunches like crazy!! Do a variety of them not to get bored. Learn to LOVE push ups, do tons of those too; do raised one on weights or a bosu ball, even a yoga ball. Also do raised leg ones, one handed if you can (not necessary but nice to show off lol) Chinese push ups, once again not necessary, but nice to know. Jumping jacks. And I would weight train at the gym like three nights a week.
I trained with the Navy eight years ago, my best friend is a Captain in the Army.

WE were both in ROTC for 4 years.
Eating... High protein diet...I'd go to GNC (or vitamin store) and buy supplements, the shake powders are really good with fresh fruit, light soy milk and ice! Also take a multi vitamin.
** When you join ROTC make a workout buddy, and train together, or even a group and get involved in the Raider Team, it is highly physical and fun!
HOORAH!