Parks you need your head looking at! Dieting is dangerous when you don't know what you're doing and can quickly evolve into a problem. If you wanna lose weight (which i can;t understand!) then like CM2 said cut out the extra's and do regular exercise. There's so much you need to check before you start dieting... speak to your GP! Personally i'd just start jogging and swimming more... at your age your body isn;t developed enough to use a gym!
I had to keep my weight at a certain level when i was competing and as soon as i stopped my weight doubled... regular exercise and eating healthy though is the key!
Jesus Loves You... Everybody else thinks you're an idiot!