Securing Mac OS X Leopard is not an easy task – well, at least not one that you can do in 15 minutes. You could follow some simple guideline, but this one is THE guideline.
One more security guide on this wonderful OS – put your Leopard on a chain, in a cage, and use it safely 
Being written by Apple, I don’t think anyone else could give you any deeper explanation on Mac OS X security and how to set it up.
Fetch it here
This time, I will not even try to copy the content to an article – the document is 351 pages long, you better fetch it to your drive and read it at home/work/wherever.
Source is Systems and Network Attack Center (SNAC) at the National Security Agency… 
Apple Mac OS X v10.3.x “Panther”
Security Configuration Guide
Guide Version 1.1
Systems and Network Attack Center (SNAC)
National Security Agency
9800 Savage Rd.
Ft. Meade, MD 20755-6704
DOWNLOAD the article from the NSA website, in PDF
This guide is long, boring, and could be considered as targeted to VERY secure environments. You have been warned!