To make the site ADA compliant, you have to follow the guidelines shared by WCAG (Web Content Accessibility Guidelines). These guidelines are widely recognized for website accessibility policy and are a standard for website accessibility legislation in many countries. It was created by the World Wide Consortium, popularly known as W3C.
Accessibility of the website was amongst the first issues addressed by W3C. The first WCAG was published in 1999 mentioning how to make web content more accessible to people with disabilities which includes visual, auditory, physical, speech, cognitive, language, learning, and neurological. Then it was revised in 2008 as WCAG 2.0. The latest update was made in 2018 as WCAG 2.2 that we use today.
While making the site ADA compliant, we will address the checklist items of levels A & AA along with verifying ADA compliance on a site using a reliable 3rd party tool of your preference.