With such a topic so broad we truly try to cover the basics from all angles in this group. Nothing too big or too small. Nutrition is as significant if not more as exercise is to reaching your goals so learn all you can.
You are only 12 and you are still growing, now is not the time to try and lose weight or to restrict your diet. Focus on eating a wide variety of healthy foods and exercising. You have some major growth spurts yet to come and will need good food to fuel them.
-------------- In truth, people can generally make time for what they choose to do; it is not really the time but the will that is lacking.
- Sir John Lubbock