Creating a "fresh" Git repo
In the top toolbar, click File > "New Repository..."
In the dialog window that pops up, you will be able to provide a lot of information on how to create a new repository in a new directory. If you choose to create a repo with name "git_demo", and choose your Desktop under Local Path, it will create new folder called "git_demo" on the Desktop that is now configured to track changes.
Click the "Create Repository" button to make this all happen.
Turning an existing directory into a Git repo