Developing a web an accessible site differs little from building any other. You simply have to keep a few guiding principles in mind:
Design with Accessibility in Mind
You'll save a lot of time and trouble in the end if you begin the development process thinking about accessibility from the very beginning and incorporating it into the natural development of your site. Consider all of the major types of potential disabilities:
Up to 35% of people age 55-65 have some sort of disability. If you are building a site for the mass public, keep in mind that as your target audience grows to include parents, alumni and alumni, you will see the percentage of your audience with disabilities increase.
Use Web Accessible Development Techniques and Tools
Building an accessible web site is more than just following a checklist of which HTML tags and attributes to remember. Most modern sites contain not only static web pages but also dynamic scripting, documents, and other types of rich media. In order for the whole site to be accessible, each of these sub-elements must be accessible as well.
Validate Your Code
Accessibility and assistive technology is predicated on having good, valid code to begin with. A validated XHTML document will have eliminated many accessibility issues for you. Always check your pages with one of the many validators available.
Test Your Site
Choose an accessibility checker and run all of the automated tests. These will find about half of the possible accessibility issues. To find more, you can view your page in a text-only browser such as lynx, or listen using a screen reader