HowtoForge have written a tutorial on how to use Iptables uding Debian.
Here you are:
This document will serve as a basic how-to on using iptables. I am in no way an iptables guru, but have been using it like this for quite a while. If I've made any mistakes please don't hesitate to email me.
Debian Sarge 3.1
Vanilla 2.4.30 kernel from mirrors.kernel.org
iptables administration utility version 1.2.11-10
This How-To is performed on a Debian Sarge 3.1 box, though the commands and syntax should work for any linux distro. Before you can configure iptables, you first must ensure that it has been compiled into the kernel, and that you have the proper userland utilities installed.