In Ubuntu the easiest way is to use apt-src. Assuming you have source repositories enabled (they are by default, I believe), you just type:
And the sourcecode for "packagename" will be downloaded and extracted to its own folder in the current directory.
apt-src install packagename
Not everything is written in C, in fact the repos contain stuff written in just about every language out there. The kernel and most of the system software is written in C, if I'm not mistaken.
What kind of software are you wanting to study? System stuff, GUI apps, utilities, services?