The most commonly referred-to guidelines for managing accessibility are the Web Content Accessibility Guidelines. Meeting Level A and Level AA of these guidelines is generally understood as equivalent to meeting ADA standards (which are not otherwise clearly defined).
The guidelines are organized around four principles:
Perceivable: Information and user interface components must be presentable to users in ways they can perceive. This covers text alternatives for images, audio and video; making content adaptable to different layout needs, and making it easier for users to see content.
Operable: User interface components and navigation must be operable. This covers improving navigation, including use of the keyboard for navigation and making sure that presentation of content does not cause seizures or other physical reactions.
Understandable: Information and the operation of the user interface must be understandable. This means making sure content is readable, that web pages operate in predictable ways and that users are helped to avoid and correct mistakes.
Robust: Content must be robust enough that it can be interpreted by a wide variety of user agents (current and future), including assistive technologies. This includes ensuring that changes in page status are clear to all users.
The W3C provides several resources to help understand accessibility needs and the stories of users facing potential barriers to use.
How People with Disabilities Use the Web