how do i make plugins?

Since you didn't say where exactly you want to make plugins, I'll assume it's for both the ClassiCube client and MCGalaxy.

ClassiCube - https://github.com/UnknownShadow200/ClassiCube/blob/master/doc/plugin-dev.md

MCGalaxy - https://github.com/UnknownShadow200/MCGalaxy/wiki/Scripting