http://freespace.volitionwatch.com/fsscp/dev/gettingstarted.php <-- Contains instructions on how to set up WinCVS to download the source code.
You'll need to know how to program in C, though, and you'll need a compiler to really do anything with the Freespace source. I don't know any good free ones myself - I use Microsoft Visual C++ - but I'm sure someone on the forum can tell you a good one.
Once you have a compiler, you should try some of the online tutorials to learn the basics of C/C++. If you like it enough, then you can buy a book at your local bookstore to learn more about a particular aspect.
You can find some tutorials on how to program in C
here, an intro to C programming
here, and a listing of sites on C programming
here.
Edit: If you've already done work with C, I'm just trying to be helpful.
