[CentOS] How to setup a computer using CentOS6 as a firewall for the whole network in my place?

> pfsense for a newbie?

Yup! Based on the simple requirements that the OP expressed, i.e. "a 
firewall for the whole network in my place", I would again recommend 
pfsense. It may seem paradoxical but it's not. It just *works* after a 
very simple and quick installation. The user only has to answer a couple 
of simple questions. A WAN interface and a LAN interface are ready and 
working together and that's it.

It can be installed on anything, from a Compact Flash card to a USB 
sticker, it doesn't even need a hard disk.