Securing Debian (and ubuntu, kubuntu etc)
There is really no point to post a step-by-step guide, write one myself, or copy it from somewhere else, when there is such a good (and BROAD) how-to on the Debian documentation project.
It applies to Ubuntu as well, and all other debain based distros. It applies to most linux distributions too, but it is better to check if there is anything distro-specific in your case.
So, here it is: Securing Debian in html format, or the same in PDF