I’ve been trying to be more accessibility conscious lately (not on this site yet though so don’t start inspecting too deeply here) and there are a lot of good resources to get started:

• Udacity has a great free Web Accessibility course by Google
• The Aerolab article 10 guidelines to improve your web accessibility is a good checklist for going through your site
• Inclusive Components breaks down some popular interface components
  
• The A11y Project is a great compilation of how-tos, tips, tests, etc. 
• If you need screen reader for testing, ChromeVox is a free Chrome extension