What symptoms of a brain tumor do you have? Remember, many symptoms of diseases and disorders overlap, you may have something less serious. However, if you feel you have a tumor, the best thing to do is to get yourself to a hospital and have an mri scan done.
Why don't your parents belive you? Have you shown them the list of symptoms and talked to them about what you are experiencing?
I think what might help you right now is to relax a bit. Often brain tumors can be removed and chemotherapy works. Try to explain to your parents. Insist that you have to see a Dr. As a start- your family physician who will refer you to a neurologist maybe.
I sincerely hope you feel better and you don't have a brain tumor. Take care and please keep posting on how you are doing.