As it's a web tool, it means you needn't worry about differences from one system to another. You simply install it and it's dependencies and then go to https://localhost:10000 to see it. It also means that if your system has no GUI (like Ubuntu Server) then you can still install and then use it from another workstation by going to http://serveripaddress:10000.
I'm running Mint which is based on Ubuntu - the steps are the same for both. This page was especially handy for me so refer to it if you want, though the below should suffice.
Go to http://www.webmin.com/download.html and download the version for Ubuntu which is the Debian based package (.deb).
Once downloaded, open a Terminal window and change directory to the Downloads folder or wherever else you downloaded it to.
Still in the terminal window, type: sudo dpkg -i webmin_1.510-2_all.deb.
(this is the general method for installing any .deb package you have downloaded, think of the command as like "Debian package install")
You will see a partial installation. It couldn't go the full way because it was lacking some perl modules and other stuff.
Type: sudo apt-get install perl libnet-ssleay-perl openssl libauthen-pam-perl libpam-runtime libio-pty-perl apt-show-versions
This will install the other dependencies. We could have run this first, of course. But then we wouldn't see the cool messages tell you that you were missing these dependencies! :D
edit: I realise now I seem to have at some point gone to this site (http://archive.debian.net/uk/sarge/all/libmd5-perl/download) and downloaded and installed the Debian package: libmd5-perl_2.03-1_all.deb
Finally, finish off the installation. apt-get has some way of knowing to fix the previous installation. Type:
sudo apt-get -f install
That's it! Now you can navigate to http://localhost:10000 from the machine you're on or http://serveripaddress:10000 if you're using another machine on the network to access the web site. You'll need a password, just use your regular system password. As you'll see, it's a pretty nifty tool. I've put a screenshot below, click it to enlarge.
|The main page of webmin, click to enlarge to full size|