Ok so first of all I'm only just learning this stuff myself so I'm no expert, but I thought I would share what I'm learning so far. It's also a bit messy but I'm just doing it as a casual tutorial for now. I will add an explination of the ball at the bottom too, and may also add more examples or update the text if you need better explinations but these things take a surprisingly long time to put together.
Anyway this is some of the very basic fundamentals of learning to shade, in this part I'm just going over value quickly, though I guess I should add an explination of all the terms and what not, but as I said it's casual for now I may add more if people want it.
I will do another part or two explaining how to apply it to actually shading something and going over a bit more information, but for now this should be of some help