How do I ensure that my website is accessible to the disabled?
Section 508 is the section of the law that requires that US government websites be accessible.
More information is here, including best practices on making content accessible to all.
http://www.section508.gov/
Generally you should support screen readers by using semantic markup, and avoid flashy content and audio -- these are usually impossible or just difficult to make accessible.
You should also look at web typography guidelines and look to hiring a good designer. Poor color schemes, typefaces, and font sizes make reading on the web much harder than it needs to be.
There are a number of considerations you need to address here, if your website isn't catering for a specific disability then you have to work on a broad range of features. In this situation the first thing you need to remember is that you sadly can't cater to everyone. Look at the list below and identify which of these disabilities you can sensibly cater for
Visual: Visual impairments including blindness, various common types of low vision and poor eyesight, various types of color blindness;
Motor/Mobility: e.g. difficulty or inability to use the hands, including tremors, muscle slowness, loss of fine muscle control, etc., due to conditions such as Parkinson's Disease, muscular dystrophy, cerebral palsy, stroke;
Auditory: Deafness or hearing impairments, including individuals who are hard of hearing;
Seizures: Photoepileptic seizures caused by visual strobe or flashing effects.
Cognitive/Intellectual: Developmental disabilities, learning disabilities (dyslexia, dyscalculia, etc.), and cognitive disabilities of various origins, affecting memory, attention, developmental "maturity," problem-solving and logic skills, etc.
The easiest here is the Seizures, eliminate flashing / strobing content from your site, or more importantly if you cant put up a warning before displaying this type of content.
Users with Motor / Mobility issues may have problems interacting with content on your site that requires a high amount of precision, this can be helped by increasing the size of your UI elements, or allowing the user to resize these elements if needed.
Generally make anything clickable as large as is feasible and if you have elements that have features such as drag drop, make the drag handles large so the user doesn't have to click a tiny area.
Auditory is also a fairly easy consideration to make, at the least simply provide text alternatives to any media content your site may have, for larger sites using video then considerations such as sign language may be an option.
Visual is probably the most common consideration web developers need to make. Firstly partially sighted users may want to increase the text size to your page, so make sure that your UI can cope with this. Use clear and readable fonts and make sure there is contrast between the background color and the font color.
Color blind users may wish to change your site color scheme to meet their needs, you can find information easily on the types of color blindness and develop a couple of alternative CSS styles to meet these needs. Also a high contrast option for everything on your site may benefit partially sighted users.
Cognitive / Intellectual is one of the harder considerations to meet, so look at the individual disabilities. ADD for instance makes it hard for a person to focus and makes them easily distracted, considering this think about advertisements, they are designed to distract us and draw our attention, thus by limiting advertisements on your site you can get rid of the ones that flash and scream Click ME!.
Dyslexic users may struggle with reading huge chunks of text which also fits in with considerations for partially sighted people, here you could have an audio option so the text is read aloud to the user.
One more consideration here is the use of color in your website. It has been proven that certain colors can stimulate emotions, for someone with emotional or developmental issues using colors that are considered calming vs ones that excite (reds for example) may improve their experience of your website.
All of the above are design considerations, looking at the development (Code) next there isn't too much you can do, most of the considerations about your code are because of third party applications interacting with your site.
Generally make sure your code is well formed, correct tags / closing tags etc. Make sure it is valid HTML / XHTML / CSS etc if you can validate to the strict standards it wont hurt your cause. Tags such as links / images should have appropriate Alt text to describe what the element is, for instance alt="image1" is fairly useless to a screen reader but alt="Image showing ...... clicking this will take you to....." is useful.
If you can find some trial software grab yourself a screen reader, load up your website, close your eyes and try interact with it, its going to be hard but at least you can see how your user will interact with your site and more importantly you can use the screen reader to check your site actually gets read the way it should.
There are plenty of 3rd party plugins you can integrate with your site to aid your users too, so look into those, things like the option to magnify text or read aloud with just a click will be well received as long as they are not too intrusive to your non disabled users.
Helpful links
http://www.w3.org/TR/WCAG10/ The W3C Disability guidelines are a good place to start http://en.wikipedia.org/wiki/Web_accessibility Wikipedia web accessability http://www.etre.com/tools/colourblindsimulator/ Allows you to see how images will appear to colourblind users http://colorfilter.wickline.org/ http://www.w3.org/WAI/ W3C Web Accessability Initiative Guidelines