A good starting point for learning about the accessibility requirements is the Jisc Meeting accessibility regulations.
The starting point for accessibility testing is to run one of the available tools on the website, such as:
Tools, such as these, are unable to test all accessibility requirements, but they generally give a good starting point. The tools, commonly, give an indication of the sections of the accessibility requirements that cannot be tested automatically - allowing these to be manually checked.
Testing Accessibility During Continuous Integration
We use the Pa11y CI tool as one part of our continuous integration (CI) pipeline. This allows us to automatically test any changes we make to our websites. We deploy a test version of the website in the CI pipeline and use Pa11y CI to automatically check many of the accessibility requirements.
A common problem for websites is the selection of colour combinations with suitable contrast. We have found Color Review to be a good way to see instantaneously how a colour combination looks.