Hrm let's see. I absolutely know jack about this stuff and i can barely tinker in some stuff, but if you actually study unlike me you'd need to know java, not javascript. You would need a text editor usually regardless of what language you code in, and also you'll need a compiler for java code. You can either attempt using an IDE, integrated development environment or something, which has both the code editor and the compiling included in one thing. Or you can use a separate set of tools like notepad++ and apache ant to compile.
The client itself compiles, or mashes together a bunch of separate tiny files that each do their own set of things, and you can find the client files either on the salem repository that JC set up a while back or one of the other custom client repositories and modify from there.
Edit mid-writing @ Kand, didn't JC set up a repo of their own? I thought dolda hasn't been updated in ages. Guess i'm confused about that.
Either way good luck, for a beginner coding is a *****, or maybe i just suck at it