Well first of all I want you to know that it is a wonderful thing, that you want to earn the title of U.S. Marine.
Though you are 15 yrs old, I would recommend that you start showing your commitment to becoming a Marine by working on becoming more physically fit. I am not going to sugar coat it for you, you are going to have to make some sacrifices and it is not going to be easy but you must mentally push your self. Once you have gotten into better shape it will all become a lot easier to you, but you got to go through some pain first.
Losing the weight actually starts with sleep. You need to get at least 8 solid hours of sleep a night. That means no staying up all night watching T.V. and playing video games. Though it may not seem like it, your body does a lot of recharging when you sleep. Try to get on a schedule with it.
Eat breakfast. This actually is used a fuel for the start of the day. If you live close to your school, (within a few miles) jog, run, or even ride a bike. They are all methods of exercise and will get you blood flowing. Do either or on your way home from school.
Working out. Start off with some type of cardio exercise before you work out. This warms up your muscles and makes them respond better to lifting weights. When you lift weights dont lift heavy weights, this builds muscle mass which makes you heavier, you want to lift light weights with a lot of repetitions, increases blood flow and burns fat which decreases weight.
Try to exercise at least twice a day. Play a sport in school like football, basketball, baseball. Its a lot easier to exercise when you are doing something that you take interest in or like to do.
Eat at least 6 times a day, but in smaller portions. Dont eat to get full, but rather to satisfy or calm the hunger craving. If you feel like you are still hungry dont eat donuts or sugary foods. Each fruits and cold vegetables. The will give you the same feeling of being satisfied but in a more healthy way.
Physcial fitness is a large part of the Marine Corps believe me, but you also have to be Mentally fit to be a successful Marine. Do this by learning all that you can throughout your schooling because it will be applied when you earn the Title of Marine. You can also go to your local recruiting office and let them know of your intentions to join the Marine Corps once you are of the legal age, and that you would like it if you could work out with the Marines or even attend their monthly scheduled Poolee functions so that you can prepare yourself for the challenges of Recruit Training.
Nothing in all the world is more dangerous than sincere ignorance and conscientious stupidity.
~ Dr. Martin Luther King Jr.