Body appearance and health is 30% working out and 70% what you eat. Heath goals differ for everyone, and keep in mind body type when you envision your "fitter" you, you want to be realistic in aims. Now that's not to say don't dream big! It just means don't be super disappointed that you don't have your minds-eye body, especially when at 14. True "six packs" that we see everywhere can take years to get. If it's something you truly want, then the only answer is hard work in exercise and discipline in food management.
Check some of my earlier posts if you want some other good food management tips.
Hope that helps:)
Diet and exercise. Eat more fruits and vegetables. Stay away from soda and even diet soda. Do body weight exercises like push ups, body squats, pull ups, dips and sprinting.
I can't add more to this than a good personal trainer could-good luck!