The suggestions above were meant for Debian.
You now have Ubuntu installed, however, and in the Ubuntu repositories there is no Webmin package. You will have to download and install the Debian package from the Webmin site manually. But beforehand you should make sure to get the required dependencies installed.
From what I can see, the following course of action should work:
On Ubuntu, just as on Debian, make sure that in /etc/apt/sources.list the lines ending in 'universe' are not commented out. Then do
$ sudo apt-get update
$ sudo apt-get install perl5 libnet-ssleay-perl openssl libauthen-pam-perl libpam-runtime libio-pty-perl libmd5-perl
$ wget
http://fastbull.dl.sourceforge.net/project/webadmin/webmin/1.510/webmin_1.510-2_all.deb$ sudo dpkg --install webmin_1.510-2_all.deb
(Of course, you may also download the webmin package any other way you see fit, wget is just a convenient CLI option.)
