You don't even need to get a firewall. Just go to your security settings in your System Preferences and enabled the built in Firewall. Also, about the anti-virus program. I do partially suggest you get one for the sake of the Windows users out there. Even though Macs aren't effected by viruses, it is still possible for you to spread the virus files through email, to Windows users who CAN get infected. Just a little notice. It's not manditory, but it's a nice addition.