If you don’t know, run your web site through WAVE and it will give you an idea how accessible your site really is.
If you don’t think accessibility is important, just ask Target what they think about it after they lost a legal battle for $6 million dollars back in 2008.
You probably want to know about 508 compliance. Here is a good article on why web designers should consider 508 compliance. Here is a 508 compliant checklist.
Now after you did all that and made the appropriate changes to your site. Run your website through WAVE again.