What is the exact physical path to your root directory on the webserver?

The path to your root directory is:
/home/username/

Obviously, substitute "username" with the login you were assigned when you first received your account details. Also, keep in mind that when you configure your CGI, that your /public_html is included in your path. For instance, if you have a guestbook.pl script in your /cgi-bin, the exact physical path to that file is:
/home/username/public_html/cgi-bin/guestbook.pl