Your phone is secure out of the box. I'm sure someone will come to this thread explaining all the details but the short answer is that there is no need for anti virus.
Windows Phone doesn't have any viruses, so you don't need an antivirus app (and there even aren't any antivirus apps in the Store). However, please be careful when downloading apps from the Store, as some third party apps are of malicious nature and may try to collect personal information.