Barb135 gave you good advice about exercising to help you with those problem areas. That Christmas dance is not far away. So besides the exercise, you might want to think of a slimming undergarment to get into that tight dress.
Take care and have fun at the dance.
At 14 and already "skinny", it doesn't like you need to lose weight, and you can't spot lose or gain. Instead of trying to lose weight, why not spend the next month working on toning those muscles? Yoga is one of the best types of exercise for toning muscles. There are tons of videos on YouTube that show you how to do it. It teaches balance, breathing and stretching. Try spending 30 min/day at it. It makes your muscles feel long and lean.