For the Git Challenges you just have to follow the instructions closely. From how to create your c9 account to completing the tutorial.
For that you will need
git-it to go to the challenges, and
git-it verify to check that you completed it correctly. For detailed instructions read them here: http://jlord.us/git-it/index.html
After you learn how to use git, here are other resources to help you learn more about git: