This article expalins how to install webmin on a Ubuntu 12.04 server. Obviously there are a lot of other aritlces on this topic, so I'll be concentrating on troubleshooting.
I prefer to use the apt-get method of installation for easy updates, and not having to worry about dependencies.
Install webmin using apt
First you have to edit your /etc/apt/sources.list file. I assume you are familar with vi.
sudo vi /etc/apt/sources.list
and paste the following
deb http://download.webmin.com/download/repository sarge contrib deb http://webmin.mirror.somersettechsolutions.co.uk/repository sarge contrib
Then fetch and install the GPG key with which the repository is signed, we will be using the root home dir to grab the key.
sudo su cd /root wget http://www.webmin.com/jcameron-key.asc
apt-key add jcameron-key.asc
Now install webmin
apt-get update apt-get install webmin
You can now connect to your webmin install using any user in the sudoers file at http://server_ip_address:10000