You should first try howto at http://www.ehcp.net/helper/yardim/cats.php
Below are extra explanations and alternate ways.. (ehcp)
Download Ubunto server 8.10 from here http://ftp.uni-bayreuth.de/linux/ubuntu/releases/
Install and do not choose LAMP server, but install OpenSSH-server (so you can do the rest from your desktop computer via Putty).
After install log in with username and password.
Become administrator with: sudo su
Set admin password with sudo passwd root