Well, first off how tall are you?
Secondly, why are you so worried about how you look? You are 12. Your body is still growing. If you get crazy cutting calories then the body cant grow to its full potential. If your weight is a problem, look for factors other than food (You need lots of food, if you havent gotten that yet ). Do you drink soda? Switch to diet soda, or better yet water.
Do you eat chips, fries, fried foods? Limit the amount of those foods (couple times a week).
I would bet you anything that just laying off of soda for a month will make you feel a hundred times better. One you combine that with eating well, you will be a machine. Muscles and weight shouldnt be an issue for you at this point. You need to focus on staying healthy and active, then that stuff will take care of itself.
Go out and have fun this summer 8)