The book on the Git website (Pro Git) is an easy read (at least to begin with): http://www.git-scm.com/documentation

A very basic tutorial can be found here:

http://try.github.com/

Can't think of a better one than Git Magic[1]. It is well written and covers really useful scenarios.

[1]: http://www-cs-students.stanford.edu/~blynn/gitmagic/

Git Immersion — http://gitimmersion.com/index.html