Honey, I hate to break it to you, but since you re 12 you re probably just growing/filling out/maturing/etc and other than maintaining active and eating decently, you can t do much to change that. Your beautiful jeans aren t going to fit you (most likely) into adult hood. Just stay positive about your body and buy new pants as needed. This question would be different coming from a 20 year old women, but you re still a young girl so it s not your fault for having fat hips/thighs, it s just nature taking its process.
do squats