How can I loose this weight?

I'm a 15 year old girl and am really overweight. My stomach hangs out over my jeans and my butt is like a huge glob of flab. Plus my thighs are massive so I can't wear shorts cause they look awful. I feel absolutely awful at school because almost everyone else is really thin and good looking but I'm like a puffy, fat blob. I absolutely hate it! How can I loose all of this horrible fat?!?!