There are many, many articles on configuring particular software or operating systems for security. The quality varies widely, and most of them miss out one or more important points. Perhaps a better approach might be to start with the basic concepts, and then apply those to the system, rather than just working with lists of tweaks. Here is an attempt to define some of the basic technical issues of computer security, in plain language.
An introduction to the security facilities of Open Source UNIX-like operating systems, focusing on Linux distributions.
A tutorial on how to configure a fresh Linux installation for greater security.