What if a 11 year old girl is over weight but also taller than what she should be
To a very high degree, you should weigh as much as you feel happy with weighing.
As far as a "healthy weight", that's what most of the other posts are about (calculating your BMI, diet plans, etc).... But most people will say what the "average" is according to different opinions.Your weight between 113.5 pounds and 150.5 pounds for normal Body Mass Index.It is much easy to calculate your weight through this Weight Loss Calculator.
As long as you are at a weight where you are healthy, the most important part is that you are HAPPY with your weight... Not that you "should be X pounds", but that no matter what you weigh, you are happy with that instead of striving to be a size 2 or something like that
That being said, if you want to lose weight, exercise is a really good (and safe) way to lose weight. In addition to that, I would recommend meeting with a nutritionist or dietitian to see if there is something else that may benefit you more.