Check the tutorials section for stickied guides, get familiar with assembly and maybe actionscript 3, check out rabcdasm, sothink, yogda maybe, and also look how the hacks are made. Look for the old mod selectors and look at the mods folder and the stuff inside those, they insert code at a line in the decompiled client then recompile it.
You can also do packet stuff, which I personally like better, you can get realm relay and get acquainted with javascript and look at some of the packet mods and stuff people released.