do i need anti virus for macbook air .

Opinions will vary. A person reasonably familiar with their machine, OS X, and basic safety (downloads, installs, etc), will not need any additional protection to that which is built-in to Mountain Lion. If you are running Windows, you will however want protection for that.

Read this tip.

https://discussions.apple.com/docs/DOC-3342

Yuk Hing:

You don't really need AV software, but if you want one, get Sophos. It's free, reliable, and effective.

Do NOT get any sort of "AV" software that is advertised to you in a pop-up ad.