Hiya Josie,
Your only 13, at this age you are still carrying "puppyfat".
Try and make yourself more active, i.e: instead of going home and going on your computer, clean your room (it might shock your parents but you will be active and in time will help with your metabolism). try and drink water, or juice instead of coke, diet coke is ok in moderation, but juice is much better. try doing different activites such as ice-skating, bowling, going for a walk with friends. The worst thing you can do is skip meals, this will not aid weight loss and may actually prevent it! Get yourslef into a routine, have breakfast when you wake up, this doesnt have to be cereal, try a smootie, you can use whatever fruit you like, add some orange juice so its not too thick, or milk with strwberries and bananas. try to limit snacks, such as crisps, chocolate, fast food. Try not to diet, your too young.. Just eat sensible, apples, oranges, salads, jacket potatoes etc..... and drink lots of fluid, especially water as this will help flush your system....
good ;luckx