IP tables is a kernel feature that lets you control the flow of IP packets.
you can use the iptables utility to set the various rules, but before you do that you should have a bit of understanding of what IP filtering is all about.
A very popular front end to iptables, is SHOREWALL. Google to their site. There are a ton of tutorials there on how to set up simple firewalls, NAT translating, forwarding etc.
I can't say off the top of my head, but I think that FC3 comes with shorewall.
There are others also similar to shorewall. Firewall, etc. etc.