You don't really need an Anti-virus app. But there are a few apps which maybe fake versions of popular apps and act as malware, or ad spamming. Try LookOut Antivirus or Avast. Both are excellent and come with Anti-theft features too. They scan apps frequently, and also after you installl any new app. So it's rather good.
You could install Prey Anti-theft for your device, It's free too.