Before you decide to apply for disability get a copy of your Social Security statement. It is a record of your earnings for all the time you have worked. It also estimates your benefit amount.
The reason it is important to get a copy of this is because this decides how much you will earn on disability. It is important to correct any discrepancies prior to applying for benefits.
Below is a link to the page where you can order your statement online. You can also go to your local office and request it.