When starting to use git, you should set your username and emailaddress, which is stored for every commit. The git website contains a great guide on getting started. Execute the following two commands to set your commit information globally for every project.

$ git config --global user.name "John Doe"
$ git config --global user.email johndoe@example.com

If you want to set the information for a specific project, execute this command without the –global flag in the repositories folder. This will only locally change the information.

On another note, GitHub also has a great guide on getting started with the git basics.