Call the College Board's customer service at (866) 756-7346. Customer service should be able to tell you if you have grounds for an appeal. If you're a special education student and you weren't given the proper accommodations, you should appeal your test if you feel you didn't perform as well as you could have with the accommodations.
Write to the College Board at 45 Columbus Avenue, New York, NY 10023-6992. Explain why you believe your score is incorrect and why this is detrimental to you. For example, if your score prevents you from applying for early admittance or if you need the National Merit Scholarship in order to pay for school, you may have a case. Make sure you include a valid reason for why you can't retake the test.
Request the College Board send you a copy of the test you took, your original answer sheet and the answer key. This lets you see if you made an error when filling out the grid. If you didn't make an error, you have a great study tool for improving your weak areas. Be prepared to pay a fee for these documents.
Appeal either the entire test or a single answer. If the College Board agrees, your entire test will be scored by hand and reviewed by a panel. Again, you'll be charged a fee for this process. If the test score was the fault of the test company, such as a processing error or failure to provide accommodations to a special education student, the fees will be refunded.