WOW! No replies to this one... DO NOT go with some company over the internet. Find someone local you can talk to face to face.
From the start I was told that I would needed a legal firm behind me. They let me know how things worked at Social Security and gave me all the links to follow the process.
The wife and I filled out all the SSI paperwork online, did an interview with Social Security and just over 2 months later I had electronic deposits in my bank account.
What I want to point out is this... SSI sent my Lawyer his check for his services at the same time they sent me my check. There was no you better pay me after you get your check routine. I had nothing to worry about. SSI did it all for me and took care of my lawyer too.
There are a LOT of SCAM sites out there ready to take advantage of you. Find someone that has an office you can drive to. Someone local who handles this type of Law.