Do you know long it generally takes to loose 15 pounds? And weight loss tips please!?

I am only 14, I'm 5'5 and I'm a size 7(125 ibs)! Thats absolutely huge! I go to the gym 4 times a week, i definitely eat very healthy, and i don't eat too much at all. My parents aren't very big, so why am i a size 7!? I would like to loose 15 pounds so do you know any effective weight loss tips (no diets such as atkins though, i want to keep it of.)