The average height for a 13 year old female is 63 inches (5' 3") and the average weight is 105 lbs. That doesn't mean you should feel pressure to lose a bunch of weight. Keep in mind, it's only an average, and your weight depends on your height and proportions. I was really obsessed with weight a couple of years ago. Since then, I've grown 7 inches and haven't gained any weight. Just don't worry about it.