We have an excellent training team at GitHub, and they've produced a series of videos talking about the basics of how to use git and GitHub.
The youtube channel is here.
We've also got several guides about how to use various features of GitHub. You can find that here.
- tryGit is browser-based tutorial walking you through using git on the command line.
- Git Immersion is another free online course that walks you through a series of tutorials, this time using your computer.
- The "Pro Git" book is a comprehensive look at all the features of git, and best of all, it's available online for free.
- Think like a git has a series of articles that help explain the various concepts you'll come across when learning git.