The only real languages you'll need to be familiar with are C++ and LUA to work server-side. If you want to make tools, most are written in C#. Nothing is in PHP besides some web applets.
It really depends on what you want to do. We don't have "getting started" guides, (but we will eventually- just haven't had the time to make them) so most people just find stuff they want to mod, then try to mod it, and if they need help along the way, they ask. I'd recommend using SIE to browse around the file and learn the various file types and how they interact with each other first.
One of the finished and very thorough guides is Adding A Planet To Core3
. So just for fun you may want to try adding a planet. SWG has 2 planets that were never added to the game, Tanaab and Umbra. Maybe try adding one of those just for practice. That will get you some experience working with Core3 and the client. Once you get familiar, maybe try making an entire new planet from scratch and populate it (that'll be more work) but you'd learn a lot.
Another simple thing you can make (and perhaps easier) are item swaps
. The SWG Client reads the TOC and TRE files, however, files that are directly placed in their appropriate folder "ui" or "appearance", etc. in your root SWG Client folder instead of packed inside a .tre override the files packed inside a .tre. So you can replace for example composite armor with mandalorian armor by taking the mando SAT files and renaming them to replace the composite SAT files. (Browse some swaps already existing for examples).
As always, if you come up with questions, feel free to ask on the forums or pop into IRC. Just play around and have fun! Try to break stuff, try to fix stuff, make it better- or worse and you'll learn how everything works.