You need to relax--the vast majority of all 'growths' are non-cancerous. In fact, it's most likely just a cyst or something. Also, at your age, the chance of you having breast cancer is astronomical... your breasts aren't even fully develped yet. And finally, even if it is cancer (which it isn't), you'd be better off to get it diagnosed now than later.
If you don't want to tell your parents, is there a walk-in clinic or something you can go to? Or another relative/aquaintance you can get to help you? Or you can just tell your parents you're not feeling well without being specific about what's bothering you, and then just confide in your doctor. They are required to keep patient confidentiality.
Good luck! I'm sure you will be fine. 