Hi Nancy,
Macs don't really need any sort of anti-virus software - that's part of their appeal. The only time Macs contract malware - trojans specifically - it's usually linked to some sort of pirated stuff.
That being said, I did a little research for you and if your Mac does happen to contract something, you could try
Clam XAV - it's free and you don't have to use it often, just when you suspect there's a problem. The Clam software is the only one I found that received consistent praise from Mac users.
Hope that helps - let us know!