Discussion on:

Utilizing the advanced firewall techniques in iptables

It is very easy to secure your environment with a Linux Firewall using IPTABLES. There are so many step-by-step procedures available online and I found the CENTOS Wiki is one of the great resources I found http://wiki.centos.org/HowTos/Network/IPTables

Follow the above article along with the above documentation for easy configuration and implementation.