If you want to play with shaders in Minecraft, here's the simplest possible shader to get started with (it just inverts your screen), along with some hints on how to make your own.
I realize this readme is lacking in detail... Hit me up with questions in the Issues and I'll fill in what's missing!
You'll need Minecraft, of course, and some sort of shaders mod. I recommend Optifine, as it has a shaders mod built in.
Once you have that all set up, find the shaderpacks folder in your Minecraft resources folder on your hard drive and drop the Inverter folder into it. Run Minecraft and select Inverter from Optifine's shader menu.
Take a look at Inverter/shaders/final.fsh and play around! The language you're using is called GLSL. http://thebookofshaders.com/ is a good introduction (though it's not specifically about Minecraft). After each change you make, don't forget to save. You can try it directly in Minecraft without having to restart the game; to get the changes to take effect you can hit F3 + R (part of Optifine) or switch to another shader and back again.
If you want to take it further, @saada2006 has a great tutorial series at https://github.com/saada2006/MinecraftShaderProgramming. You can also examine the code of more advanced shaders, such as Chocapic13's Shaders.
Learning anything is easier with a community. The shaderLABs Discord server is a vibrant place to connect with other shader programmers, both novice and experienced.