How can a 12-year-old lose weight who is 5'2'' and 105 pounds?

The healthiest way is to cut your calorie intake and start exercising. Diet pills work, but are not good for your body and their effect is only temporary. If you want the weight off for good, do it the old fashioned way and start being more active. Take up sports and be outdoors as much as you can. Being indoors will temp you to eat and sit in front of the TV. Good luck!