At 12 years old she needs to grow into her weight, not worry about losing any.
Lots of exercise and a balanced diet of 10-15 calories per pound should get her on the right track. She needs protein for growth, fat for good health and proper digestion, and carbohydrates for energy and insulin boosts which promote growth.
Actual restriction dieting would be a terrible thing for her right now, she needs her nutrition for the next few years for some very important growth. Brain growth, hormone function, muscle growth, strong bones and teeth...all of these come from the food she eats.
If she's interested a
moderate weight training program might be good, even with bodyweight.
Lots of cardiovascular activity to support those sports, running, sprinting, bike riding, even long walks are good. Make it fun and exciting and she'll be ready to participate.
My 11 year old gets a little chubby in the winter when her only sport is basketball and she can't get out to play as much. Then in the summer working out for softball and getting in shape for the fall cheerleading season trims her right up with all the running and bike riding she does, add a few miles of walking up and down the creeks while fishing with dad a few days a week, some swimming... The difference is in the exercise because she gets pretty much the same balanced diet all the time.
Get yours interested in more physicial activity and find a fun way for her to train for it and she'll be very happy with the results without getting an "I'm too fat and need a diet" complex.