I am 13 and I am getting stretch marks on my legs I know I am putting on weight but I don't know how to lose it?

In order to lose weight you must "spend" more calories than you take in. For example, if you eat a chicken sandwich from Popeye's you will have consumed about 680 calories. Walking will burn about 114 calories per hour. You see how much you would have to walk to get rid of the calories you just took in. The solution is a low calorie and low fat life style change. Don't call it a diet because "diets" don't work but life style changes are forever. Please remember to take in 64 ounces of water a day. This will help to flush the toxins and the fat from your body.