Have you read thru the sticky threads of how to start a claim and tips etc?
That is a great place to start and will help you sort out if SSDI is feasible in your case or not.
here are the links for those-
http://neurotalk.psychcentral.com/thread148967.html
http://neurotalk.psychcentral.com/thread120255.html
http://neurotalk.psychcentral.com/thread367.html

Exploring previous threads will be helpful too, to read what others have experienced.