Newcomers to APIcast development might be interested in the following resources.
At first, you might need to learn the Lua language (with a focus on version 5.1/5.2). The Lua 5.1 Reference Guide should be your first read and followed by Programming in Lua.
Then, we suggest learning Nginx with the agentzh's Nginx Tutorials.
You might then learn OpenResty by reading its documentation, full of examples and going through the Getting Started Tutorial.
As you start writing your first APIcast policy, you might be interested in the OpenResty resources, that present real world use cases and their implementation using OpenResty.