Quick Github guide and contribution guidelines.
http://git-scm.com/docs/gittutorial THE git tutorial. A decent introduction.
This site also hosts the free, online book Pro Git, which, despite it's title and depth of content, is remarkably readable and a good beginner book.
Atlassian makes my standard git-gui, SourceTree. Their git tutorial is good, but can use jargon without properly presenting it. https://www.atlassian.com/git/tutorial Their Workflows overview https://www.atlassian.com/git/workflows is a great look at the variety of ways you can use git to manage your project
http://gitimmersion.com/ Commands and output you can try on your own. http://rogerdudler.github.io/git-guide/ A "comic-style" overview ... just keep scrolling.
https://try.github.io (Code School)
https://github.com/edx/edx-platform/wiki/How-to-Rebase-a-Pull-Request
https://help.github.com/articles/what-are-other-good-resources-for-learning-git-and-github