Making your website accessible is very important. The Web Content Accessibility Guidelines are the definitive guide to making your site accessible. There is also a checklist based on the guidelines produced by WebAIM.
Making your site accessible helps everyone, but especially users with visual or hearing impairments, and people with slow web connections.
A useful resource for understanding accessibility issues is Dive into accessibility, which presents a series of things you can do to make your website more accessible, and some case studies of people with disabilities.
There is also a selection of tools you can use to analyse your web pages for accessibility. These make the process very much easier - but they are no substitute for understanding accessibility and learning the basic accessibility guidelines.