First, you are going to be interrogated by a representative of the Social Security Administration.
I know that the word interrogate sounds extreme but to most people the interview seems like an investigation. The total stranger that is going to ask you very personal questions is a totally unbiased prospector for the facts. It is best that you expect this process to be unpleasant. If it all goes smoothly and you feel like you've been visiting a long lost relative then that's a bonus. However, you need to go into this interview poised and prepared. Don't expect the interviewer to think your case is special because he/she has done this a ton of times and they've pretty much heard every type of story.
Secondly, have your financial records in hand.
These records will include this year and last year's income amounts, this year and last year's tax forms. You should have a complete and detailed accounting of all of your expenses too. Not only will you need to provide all of these details for yourself but you will need the same records for your spouse. Have contact information such as your employer's name, phone number and address. The SSA will not just take your word for anything concerning this application process.
Thirdly, have your complete medical condition documented.
You will want to have a complete record of the dates of doctor's visits and testing evaluations. You will want to include notes from your doctor. It would also be a good idea to have a second opinion concerning your physical inability to function in the work place. Just because your doctors think you are disabled does not mean that the SSA will think you are disabled. Learning how to get disability benefits requires preparation.
Fourthly, be prepared for rejection.
Seventy to seventy-five percent of all first time disability applicants will be denied benefits. I say this to stress the need for persistence. Rejection is the normal process and nearly everyone who has gained disability benefits has had to file at least one appeal and often more than one. So don't lose heart and continue to fight for disability benefits.
If you feel you have a need that could be met by SSI or SSDI please check out the free report at http://www.howtogetssi.blogspot.com or you can pick up a free resource kit at How To Get SSI.