That's a very complex question for a simple answer.
You may want to take a look at existing API frameworks, like Swagger Specification (OpenAPI), and services like apiary.io and apiblueprint.org.
Also, here's an example of the same REST API described, organized and even styled in three different ways. It may be a good start for you to learn from existing common ways.
At the very top level I think quality REST API docs require at least the following:
Also there are a lot of JSON/XML-based doc frameworks which can parse your API definition or schema and generate a convenient set of docs for you. But the choice for a doc generation system depends on your project, language, development environment and many other things.