The most useful thing you can have is an official medical letter stating your diagnosis and giving a brief explanation of the ways it is likely to affect you (I've found mine so useful that I've photocopied it and make sure that I have a copy with me at all times). If you can anticipate in advance any special requirements you will have, ask your clinician to mention them specifically in the letter. Check out this sample university accomodations letter for an autistic student on Jim Sinclair's site.
(If you haven't been formally diagnosed, check out the page of diagnostic resources).