Level A guidelines
Level A represents the lowest level of conformance and includes the most basic of web accessibility features. Every page on the internet should start by meeting this basic level of conformance.
Once you have gone through these, make sure you check out all the level AA guidelines as well.
To be clear, accessibility is not a checklist or a tool, and doing everything exacly as documented here will not always guarantee an accessible, or compliant site. It's important to note that how you deliver accessibility is unique and completely dependant on your particular site content, and users.
Ensure everyone can perceive content
Based on the Perceivable principle:
Information and user interface components must be presentable to users in ways they can perceive.
Color
Semantics
Static content
Images
Audio and video
Ensure everyone can interact with content
Based on the Operable principle:
User interface components and navigation must be operable.
Ways of navigating
Ways of interacting: Keyboard and focus
Ways of interacting: Touch and mouse
Animations
Time limits
Ensure everyone can understand content
Based on the Understandable principle:
Information and the operation of the user interface must be understandable.
Content
Consistency
Context switching
Forms
Ensure content is robust
Based on the Robust principle:
Content must be robust enough that it can be interpreted by a wide variety of user agents, including assistive technologies.