, you seem to have bitten off a little more than you can chew, for now. May I suggest that you first focus on getting the ordinary elements of Linux networking up and running, and only after that has been accomplished, set up the firewall/router component. To do otherwise will lead to confusion and problems.
Since you are just starting out, it makes a lot of sense to leverage the expertise and development history that comes out of a GUI-built or canned package designed to produce a solid, working firewall. I found numerous links that seem to have promise by Google searching for 'iptables firewall builder'. I can also recommend the homeLANSecurity
package, which comes as a small collection of structured and documented shell scripts that build your iptables
firewall. It is built to fit the LAN-WAN model that seems to match your circumstance, and is easy to install and customize.