Aaahhh, that's the hard part. I don't know much about doing it for Windows (MinGW or Cygwin, I think?), but with Linux systems, all you have to do is install the build-essential package. Mac users will need to install the Xcode developer tools.
When you're in the directory with the source code in it, simply run make. Hopefully, it won't spit out any errors and then you'll have some stuff compiled. Note that in order to make anything work with current Tremulous, you'll have to apply something called a "backport patch".