1. Our "Git Fundamentals: Basic Concepts and Definitions" webinar, http://www.verticalsysadmin.com/git/ (you have to register) lays the groundwork you need to start studying Git.

If you have a team that needs Git training, email me and we can deliver a instance of this webinar (complete with Q&A session) free of charge (as an introductory service).

2. For someone just starting out, I also recommend "Learn Enough Git to be Dangerous", https://www.learnenough.com/git-tutorial (HTML version available for free), from @mhartl, author of the popular RailsTutorial.org

3. The "Pro Git" book is great. https://git-scm.com/book/en/v2 A comprehensive resource.