Simple Minecraft Shaders
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!
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, but you do have to switch to another shader and back again for the changes to take effect.
Thanks to TheCool1James's intro for many of the links above.