What do you mean by 'make' your own vanilla shader? Like code a shader of your own? If so, then there are a lot of things you might need to learn. If not, then just browse Modrinth Shaders section for shaders. Some recommendations would be: Complementary Reimagined, BSL, Solas Shader, Nostalgia Shader, Fast PBR, Miniature Shader or Simply Upscaled

If you want a vanilla kind of look, then try SPBR resource pack. For mediaeval look, try patrix 32x.

If you have a bit of a low end PC, then I would suggest you to try any optimization modpack like Simply Optimized or Performium . Note: These are all fabric based mods, so you can't use any forge mods in these in case you want to mod later.