Securing Mac OS X Leopard
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