There's a VERY nice guide to securing Debian Linux over at linuxsecurity.com. Although it's aimed specifically at Debian users, most of the advice is applicable to any modern Linux distribution. Anybody who's new to Linux security would do well to give this document a careful once over. It's very well-written and pretty thorough, on the whole.
http://www.linuxsecurity.com/docs/ha...-debian-howto/
