This is often debated, and some will say none is needed, however, most agree if you want to run one, that ClamXav is the one to use. It's donation ware, and has been around for a number of years. I've used it for years with no issues.
Thanks guys this helped alot when i purchase the macbook is there anything that I should make sure it comes with are there any discs or anything that I need?
If it comes with OS X 10.6 (or earlier depending on year), you need to get the original OS X disks that shipped with the system. They will be gray-faced and there should be two disks, one a systems disk and one an applications disk for the MacBook Pro. The gray-faced disks are locked to the systems type they ship with, so, for example, OS X disks that shipped with an iMac would be of no use to you.
If for some reason you buy a system without the disks, you may be able to buy them by calling Apple support, if they still stock them. The original disks allow you to restore your system and is the only source for the Apple hardware test, which is handy to have.
A 2011 or newer system with OS X 10.7 Lion or newer won't have the disks, as that's all online now.