Jump right in and get an overview on using the API in your projects, from installation all the way through to making it do magic. Have things working and want tips? The cookbook provides some tips and tricks. Getting started and want some full examples? The ApiPromise examples provide some basic examples.
For oft-repeated questions, the FAQ may have what you are looking for.
Separated from the API you will also find the Substrate metadata section that has an overview of the RPC, extrinsics, events and errors available on a typical Substrate node.