The Git Book is the official tutorial about Git.
Ranked in these QuestionsQuestion Ranking
Pro Available for free online
Although the book is free online, you can support the author by purchasing a hard copy of the book.
Pro Comprehensive and clearly written
The book is a comprehensive guide on everything you need to know to learn and master Git. It starts with the basics, but also moves into the deeper complexities of Git, not only explaining what commands do, but how.
A strong understanding of the inner mechanics of Git will help you use them to perform very powerful operations in keeping track of your code history, and developing features.
Pro Available in many languages
Con May be intimidating for beginners
It's an excellent treatment, but it may not be the place a new person wants to start. (At least reading cover-to-cover.)