Nothing will be 100% applicable to AppGameKit, because the variable names will be different. But they are usually closely named.
So first start here(
https://www.appgamekit.com/documentation/guides/13_shaders.htm) to learn the variable names. Or, at least, read it, and keep in mind what AppGameKit calls the world matrix, world normal matrix, the projection matrix, view matrix, and projection view matrix. posVarying and uvVarying are also extremely important variables that are passed to the pixel shaders(a variable passed from the vertex shader to the pixel shader is called a varying ). They are usually referred to as fragPos and texCoords in a lot of tutorials.
This is my favorite intro I have come across.
https://learnopengl.com/Introduction
While Unity doesn't use GLSL it is very close, and once you become familiar with GLSL you should be able to port it over pretty easily.
I recommend this after a bit of GLSL experience:
https://catlikecoding.com/unity/tutorials/rendering/
GameGuru has lots of shaders, and Janbo's shader pack. These are all excellent resources to learn.
