Magnolia 5.3 reached end of life on June 30, 2017. This branch is no longer supported, see End-of-life policy.
The Swagger API explorer is the easiest way to learn the API. Swagger is a tool that shows what parameters and options the API accepts and how it responds to your requests. Swagger also acts as live documentation that is always up-to-date with the Magnolia instance it runs on. The tool understands both XML and JSON. You can find it in Dev > REST Tools when the REST Tools module is installed.