ANDY'S ANSWER: A growing body needs nutritious food and lots of exercise. Too many teenagers have become addicted to junk food like chips and candy instead of fruits and vegetables. Fast food like pizza and hamburgers are also loaded with fat, but seem to be what young people are living on. The bottom line is if you are taking in more calories than your body needs and not getting enough exercise to burn them off, you will gain weight.
Chances are, if the majority of what you eat is coming in a package then you are probably not getting the nutrition you need and are consuming way too much fat. The Canada Food guide recommends that teenage girls eat 7 servings of fruits and vegetables (1 piece of fruit is a serving, or a cup of leafy vegetables), 6 servings of grain products (a grain product serving is 1 slice of bread, 1/2 bagel, 1/2 cup cooked pasta or 3/4 cup of cereal), 3-4 servings of dairy or alternatives (1 dairy serving is a cup of milk or soy, 3/4 cup yougurt, or 1 1/2 oz of cheeze) and 2 servings of protein per day (1 protein serving is 1/2 cup cooked fish, poultry or meat, 3/4 cup of cooked beans, 2 eggs or 2 tablespoons of peanut butter) per day.
If you make sure that you are eating the recommended portions of unprocessed food and get at least 30 minutes a day of aerobic exercise per day, I'll bet you'll have no problems maintaining a healthy weight.