I own a mac also. I have done a TONNE of internet stuff on it and NEVER gotten a virus, or had spyware etc.
However it still pays to be careful, so I have Norton Antivirus for Mac. But there is a free program called iAntiVirus, which is just as good as Norton Antivirus, and in some cases where I tested Norton and iAntivirus by downloading a test virus, iAntivirus was actually quicker. And it's free! So dont waste money on an antivirus program, just get iAntiVirus. On the odd occassion that your mac does get a virus/spyware, iAntivirus is more than capable of removing it!
You can download it from this site:
http://www.iantivirus.com/