Right now there are no active viruses out there that attack the Mac.
There are a number of Trojans than can cause minor problems but nothing on the scale of Windows.
AV software will NOT protect your from any attempt to steal card numbers.
Best defense for that is to be absolutely sure of where you are giving your card numbers out.
There are a number of AV apps for OSX, ClamXAV is one of them. This is free.
I would not recommend anything from Symantec (Norton) or Mcaffee.
The majority of Mac users do not run any AV software at all.
All the techs tell me a MAC does not need Anti Virus Software. If not what is the best way to protect yourself online. Card numbers etc.