I found a very good Blog article from our phpBB dev colleagues.
Please check it out here: http://blog.phpbb.com/2012/01/11/how-you-can-aid-development/.
Just to mention that Joomleague is using gitorious.org as public repository instead of github.
Don´t hesitate to ask questions about git on our forum, but pls try out to
find the answer on a searchengine of your choice first! thx!