Grab the Hurd CVS tree (see http://hurd.gnu.org for details), and read the file doc/navigating. That should give you pointers to understanding the Hurd. While reading it, you should use Debian GNU/Hurd and try to find things that are broken, missing, etc. And report those, with maybe a patch. If you wish to contribute large amounts of code, then you should sign a copyright assignment for the Hurd, email@example.com is the place to ask for that.