Is it just ip masquerading (NAT) you want to get working with iptables? The NAT howto at www.iptables.org
gives a good explanation on how to get that working. If you want to take it a step further and use iptables to configure your complete firewall then you might want to read some of the other howto's at www.iptables.org
the packet filtering howto might be a good one to start with.