The path to military medicine starts when you are applying to medical school. Until then all you need to do is to not to develop a drug habit or morbid obesity. If you want to go all out you could even make a habit of running twice a week. Check back in here when you're ready to apply.
Get good grades, especially in science core and score well on your MCAT. Your last year of undergrad once you've gone though the application and are getting interviews then you can come back and start asking questions.