Navit is written mostly in C, and the code is automatically documented using doxygen. To get started quickly, checkout a copy of the source from SVN:
svn co https://navit.svn.sourceforge.net/svnroot/navit/trunk/navit navit
To see who's currently helping to develop Navit, see the Navit members and contributors page.
Patches for bugs or new features are very much encouraged, and can be submitted via Trac.
If you feel that you can provide a positive contribution to Navit, ask in IRC for SVN access - it's usual for developers to submit a few patches (to check credentials) before SVN access is granted. If/once you get access, please ensure that you have read the SVN guidelines before committing changes.
Documentation of the codebase is pretty sparse on this wiki. What documentation is available is noted in the wiki index.