quetzalrojo on "Where to begin"

Hi everyone,
This is my first-time on WP forum. I was looking for an open-source project to work on and though I would contribute to WP. My question is, where is a good place to begin learning about back-end WP issues? I'm a computer science major, I use Linux (Debian/crunchbang), and I'm hoping to build up some WP skills along the way to possibly do some freelance work. Any help is much appreciated. Thanks in advance.