By default, this command opens up a Bash session instead of a standard Windows cmd.exe session. So first you should run "Git Shell" from the start menu (Git Shell will invoke a Bash session for you). Then simply cd to the root directory of your repository.
Note that Bash uses Unix syntax for paths, unlike from cmd.exe and other Windows software. It uses forward slashes instead of backslashes, and "dives" are rooted at /. So, for example, to work on a repository located at C:\Users\SM79\Desktop\My Project\ you would run something like cd /c/Users/SM79/Desktop/My\ Project/.
Tab completion can be very helpful, especially when working with directories like My Project that contain spaces. (Typing the My and pressing Tab should complete the directory, including escapes for spaces or other reserved characters, as far as possible without ambiguity.)