These topics have been covered thoroughly in past threads. Take a look at the list of threads related to yours that are listed at the bottom of this page.
You definitely need a firewall because even though Linux is quite secure, someone can still crack into your system. Router passwords can also be cracked, so running a firewall on your computer is an important backup. As for antivirus, you do not need it, unless you share files with Windows users. Most antivirus solutions search for Windows viruses and not Linux viruses because there aren't any active Linux viruses out there.