A good place to start is by reviewing the WCAG standards. This guide gives the full compliance list, and techniques for how to fix any issues your website has. To find the problems your website has in the first place, there are a large number of services you can use; from automated subscriptions, to one-time scans, to continuously scanning plugins or modules, to browser extensions. To learn more about these testing methods, read “Best Ways to Test Website Accessibility; Easy, yet Comprehensive!”.