How can i lose about 50 pounds in 4 months?

I am 13 and weigh 250 but I am 5'11. I play basketball,volleyball and go to the gym regularly I also hav b-ball tournaments and practices 3x a week. But I only lose a little weight at a time but then I gain it back. I eat all healthy and organic foods and I do NOT eat junkfood . I just can't shed the pounds can someone help me?