Here is a link to the highly informational Social Security website:
Here, you can set up your own account that will tell you what your monthly payments will be when you retire. You can also see if you have enough "quarters" (work history) to qualify for SSDI and what your monthly benefit will be if your disability application is approved. The SSA site also has links to most of the commonly used forms that are submitted when applying for SSDI and SSI (don't know the difference? you are not alone! visit the SSI/SSDI page of this site for more information on the differences between SSDI and SSI).