rubbish on the whole 'not appropriate for a young teenager' - lets remember there are professional athletes at tender ages of 15/16 *cough* the mighty Reece Oxford *cough* that follow strict diet plans etc.
that being said Rebecca, I don't think a 'diet' plan is what is required ... go old school on it - have a cereal/porridge breakfast every morning, a sandwich with a piece of fruit for lunch and a healthy portioned dinner. one thing I've also done is cut fizzy drinks out of my diet completely (besides beer! (something I hope you wont need to worry about ha-ha)) just drinking water will make you feel a lot healthier inside.
besides this basic food plan, make sure you keep your fitness up - again nothing too drastic, a simple 3 - 4 mile run 3/4 times a week will increase your fitness ten-fold! and at your age 3 - 4 miles is ample, as you get older and reach 16/17 etc. you should be able to push a 5-6 mile run 3 to 4 times weekly.
swimming is also a great exercise, get down to your local swimming bath! ask a lifeguard how many 'lengths' make a mile ... and then try to swim a mile ;-)