I went through this process recently and I was approved for disability on the first go around, so I will list out the most important information:

It is true that a relatively high percentage of people are denied on their initial application, however this is typically not due to the person being ineligible. Rather, it is because there is not enough evidence provided for the processor to make a decision.

Another common reason for denial is that the applicant has not worked long enough. For example, when I applied I was 26 and needed to have something like 8 or 10 years of work history. The details for all of that are on the SSDI website.

What you need for a successful claim is:
A complete treatment history including records from clinicians, prescribers, hospitals and so forth.

Fill out every detail that you can on the forms that they send you. What they are looking for are your attempts to work, your day to day symptoms, treatment that you have tried, and how your symptoms affect your functioning.
The more detail the better, and it is also absolutely legal and even encouraged, to describe what your symptoms are like when they are at their worst.

You can use a disability lawyer, if you want to. All that they will do is make sure that you have all of your documents in order and they will take 25% of back pay once your claim is successful.

What will your back pay be? The processor will determine a date of disability. That is typically when you could no longer work full time. You are entitled to start receiving benefits exactly 5 months after your date of disability.

How much would you receive each month? That will be based on how much you have paid into the system, which has been accruing all the time that you have been working.

Having your the support of all of your treaters and consistent history of receiving treatment, taking meds, and so on, will help. Just send all of the documents that are requested and keep copies for yourself.

The process does take an average of 3 months. You can speed it up by having all of your paperwork sent as soon as possible. Also, don't be discouraged if you are denied on the first try. Your chances of being approved go up when you are in the appeal process.

I know this is a lot of information, but I hope that it helps. I don't plan on being on SSDI forever, but it has given me the ability to focus on treatment. It also helps gain access to resources for getting back to work slowly when you are ready.

Good Luck, and feel free to ask if you have any questions.