Questions or comments for the Git community can be sent to the mailing list by using the email address firstname.lastname@example.org. Bug reports should be sent to this mailing list.
You do not need to subscribe: you will be Cc'd in replies. Please keep the Cc list intact when replying (use "Reply to all"). Greylisting may delay your first post for a few hours. Note also that the list only accepts plain-text email; please disable HTML in your outgoing messages.
There is also Git user mailing list on Google Groups which is a nice place for beginners to ask about anything.
Bugs can be reported directly to the mailing list (see above for details). Note that you do not need to subscribe to the list to send to it.
If you are new to submitting bugs, you might find this guide helpful for producing useful bug reports.
If the manpages and this book aren’t enough and you need in-person help, you can try the #git channel on the Freenode IRC server (irc.freenode.net). These channels are regularly filled with hundreds of people who are all very knowledgeable about Git and are often willing to help.
If you need specific help about one of the for-profit Git hosting sites, you might try their own IRC channels (such as #github or #gitorious) on the same IRC server.
Contributing to Git
The Documentation directory in the Git source code has several files of interest to developers who are looking to help contribute. After reading the coding guidelines, you can learn how to submit patches. For those looking to get more deeply involved, there is a howto for Git maintainers.