How many calories should a 12 year old girl eat to lose weight?

It depends on many factors, so you should consult a nutritionist or a doctor. Keep in mind that diet is not the only thing needed; you also need exercise, because just reducing calories will cause a decrease in metabolism, and you need to exercise to keep the metabolism up at the same time you decrease calories. You also need to exercise at least 40 minutes a day, with your heart rate in the target zone, otherwise you will not tap into stored fat reserves. Again, consult a doctor to make sure you are able to exercise without hurting yourself, and to find out what the caloric intake and target heart rate should be.