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, when you configure your script, then remember to include your /public_html in your path. For instance, if you have a mail.php script located at www.yourdomain.com/mail.php, the exact physical path to that file is:
/home/username/public_html/mail.php