Documentation for Drupal 8 is still really lacking. There is SOME documentation out there, but in typical Drupal 8 fashion, you have to sift through all of the pages and all of the content to find exactly what is relevant to you. This guide is an attempt to get you quickly started.
This guide is for dummies, but it also assumes you know the basics of Drupal 8. Specifically, you should be comfortable getting an installation of Drupal 8 up on your own, you have you’re comfortable editing text (config) files, and you have basic site-building skills (Creating content types, creating views, etc).
Before we get started, I want to say that, when I am making an API for a new app, I want to make it FAST. I am comfortable working with the MEAN stack (using MongoDB + Express to run your API server), but I consider myself fairly proficient in Drupal, so why reinvent the wheel every time manually writing an API when Drupal already does a great job at assembling and delivering data? I’m able to get my API working with GET requests in less than 5 minutes, and POST requests in less than 15 minutes.
OK, enough small talk – lets get started.