Piggybacking off of @Sweatix 's post, a few other links that might help:
The asset library has some good ones, Shader Asset Library, you can check out snippets of code to explore and learn.
Which you can access in the editor directly also:
But also this forum thread about a youtube playlist of tutorials.
https://forum.godotengine.org/t/godot-4-shaders-a-list-of-video-tutorials/36500