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