Binos
Your mac comes with a built in firewall within the operating system. Go to the 'apple' (top left) - System Preferences - under 'Interenet and Network', click 'Sharing'. You can set your preferences about firewall and file sharing etc. from there.
As far as antivirus software is concerned, personally I'd keep away from Norton/Symantec. Having loaded it on originally, I found that so many onther settings had been changed (mainly due to the firewall part of it) that it became a real pain in the @rse when setting up things like iSight. It was removed. If you feel that you want the security of an antivirus, look at Intego
Intego
They seem to be the programme of choice, although you will find plenty of others out there. Personally, I use VirusBarrier. I also know a lot of mac users who don't bother with any, and none to my knowledge have ever been infected (as you say, viruses for mac are few and far between, although not unknown. Probably because it's too hard to write them!)
Trips