deb http://download.webmin.com/download/repository sarge contrib
deb http://webmin.mirror.somersettechsolutions.co.uk/repository sarge contrib

When you are finished editing, save the file by pressing Ctrl-X, then yRETURN.

Now add the Webmin GPG key to apt, so the source repository you added will be trusted. This command will do that:

wget -q http://www.webmin.com/jcameron-key.asc -O- | sudo apt-key add -

Before installing Webmin, you must update apt-get's package lists:

sudo apt-get update

Now run this apt-get command to install Webmin:

sudo apt-get install webmin