Are you asking for resources specific to an game engine (like Unity or Unreal) or more general?
Asking "how do I write shaders" with out context is hard to answer as it's nearly as open ended as "how do I write game code". If you've got a base of c++ that'll help as you'll understand the basic syntax and conditionals as both GLSL and HLSL are roughly C like.
However if you really just want to learn shaders and aren't tied to a particular engine I would suggest switching to Unity as, unlike with Unreal, writing shader code directly is a common thing to do and there are a lot more resources available for helping to learn to write shaders for Unity. Writing shader code in Unreal is a much more "advanced" use of the engine, and there is relatively speaking far fewer resources available to help learn.
This link is the one I send almost everyone to first, as it does a good job of explaining the basics, both in an engine / api agnostic way and then goes into Unity specific code.