hey genete, I'm in the same boat. It has taken a while only to know of what to start with.
I made a bit of progress starting from the GLSL documentation. It explains better the the Godot documentation some of the basic functions and concepts. Once you understand what a shader is and other basics. Go through this basic tutorial:
basics, it will show you how to start a shader project in Godot. Then go to the tutorial Gokudomatic2 pointed out and at the same time try to identify the same nodes in Godot (the tutorial is for Unreal engine, which seems to be more complex, but still you can find your way around).
Then reading the documentation from Godot will actually start making more sense.
You can always ask questions here or look through other questions tagged shader.
Hope this helps a bit.