Edit the /etc/apt/sources.list file on your system and add the lines :
deb http://download.webmin.com/download/repository sarge contrib
deb http://webmin.mirror.somersettechsolutions.co.uk/repository sarge contrib
You should also fetch and install GPG key with which the repository is signed, with the commands :
cd /root
wget http://www.webmin.com/jcameron-key.asc
apt-key add jcameron-key.asc
You will now be able to install with the commands :
apt-get update
apt-get install webmin
All dependencies should be resolved automatically.
Reference: http://www.webmin.com/deb.html