Block incoming traffic FTP from internet using iptables

Hi everybody. I have the next scenary:

eth0: WAN
eth1: DMZ
eth2: LAN

I need to block all incoming trafic from the internet through my network LAN using iptables. I have squid but i need to do this using ipatbles.

I have been listening about iptables -A FORDAWARD but I am stuck right now please help me.