If you need an antivirus, try avast! It really is one of the best anti-virus around....free of cost and provides a whole lot of features.....run by the open source community. It also provides similar levels of security for Windows & for Android too!
Give it a try & I trust you will not regret.
Anti-virus software is not necessary at this time, provided that you keep your machine updated and aren't keeping Java enabled in your web browser. Even if that's not the case, Mac malware is rare and hard to find - most is, in fact, highly targeted... oddly, a lot of it right now is aimed at supporters of the Dalai Llama in Tibet.
For more information, see my Mac Malware Guide.
BTW, I would not recommend avast! on the Mac. It's got good coverage, according to my recent testing, but that's only a small part of the story. I've seen it cause kernel panics for some people in these forums, and it has a reputation for false positives. I personally have observed that it often seems to label Windows malware as Mac malware, which makes no sense whatsoever.