This category feels empty so I'll throw this here for forum lurkers. "Shaders for Noobs" is an open-source website with a collection of beginner-friendly tutorials on s&box shaders.
Current collection should help you understand the basics of s&box shader API, starting from a primitive unlit shader, and then eventually growing it into a proper lit shader, step by step. After that, you'll learn more specific things like manipulating vertices in vertex shader, frame buffers, vertex colors, render attributes, opacity & alpha test, texture blending methods and other stuff.
There is also a category for post-processing shaders, and collection of little reference articles for things like naming conventions, explanation for vector components and normal maps, and a little guide on fixing old shaders that use deprecated API.
It is currently work-in-progress, there is no tutorial on compute and geometry shaders, but this is something I'd like to get done in the near future.