Godot 3 right?
Create a SpatialMaterial and enable basically everything you find interesting in the material. Then step back 1 level in the inspector, right click the SpatialMaterial and select
Convert to ShaderMaterial. Should give you a good starting template to learn from.
Honestly though it's best to search for
OpenGL ES GLSL tutorials since the godot ShaderLang is basically just a subset. For most/simple materials you shouldn't even need to use custom shaders, just SpatialMaterial should suffice.