What is the amount of calories a day would a teenage girl need to eat to lose weight?

It depends: The number of calories you need to eat to lose weight depends on your age and gender, how much you weigh now, and how active you are. If you're concerned about your weight, ask your parents about making an appointment with your doctor or getting a referral to a registered dietitian. Don't try fad diets that cut out a lot of foods or promise that you'll lose a lot of weight quickly.