The only "viruses" out there for Macs are actually just programs that you run, give your password to, and then watch as they do bad things.
In other words, there's nothing you need protecting from as long as you only instal programs that you trust! (Finding reviews of programs so you know they're real and only downloading them from their 'official' site is your best bet.)
There IS anti-virus software for the Mac, but the only real reason to get it is if you forward a lot of stuff to Windows users. You may get a virus and forward it on to your friends, even if it doesn't affect your computer. A scanner program could catch those for you before you hurt your friend's Windows machines.
But that only happnes if you get an odd e-mail and then forward it. How often does that happen? I usually always start a new mail message, so I can't see that being a problem for myself either.
The 2 main things you should do do protect yourself:
1) In System Preferences: Find and turn on the firewall (search for it)
2) In Safari Preferences: Turn OFF the "open safe files after downloading" option