[CentOS] centos5 iptables expert needed

Hello,
    I'm setting up a centos5 router for a friend. It will direct traffic to 
an internal webserver, already in place, as well as run squid proxy. It 
should do nat and have a firewall with iptables. I've set up routers before 
for this purpose but always of the bsd type and using their firewalls. I 
understand the concepts, nat, packet filtering, etc. but i'm not getting 
iptales at all. If anyone is an expert on this i'd appreciate hearing from 
you. I have what are probably basic questions, i've read the docs, but i am 
lost.
Thanks.
Dave.