How do weight loss pills work? Is it safe to use them for weight loss?

I’m 14 and I’m 4′6 and I weigh 68 pounds. I’m a competitive gymnast and fat is really not good so I need to lose weight. Some of the girls on my team use weight loss pills, and they say they work really well. Oh and also are they expensive? Do you need a prescription?
Exercise and diet aren’t the problem. I train 6 hours a day and eat about 1300 calories.