Firewall/Networking Question

Greetings, I apologize if this isn't in the correct group.

I'm working on my home network and wanted to add a firewall in the mix
of things.

My set up is looking like this:

Cable Modem --- Firewall --- Linksys WRT54g --- Computers

I can't get this scenario to work for the life of me. The Firewall is
on a three leg network, one to the internet with 4 static IPs, one
going to my internal LAN and the other going to a DMZ.

When setting up the internal network I was specifying the Firewall as
the gateway, 10.12.1.254, my router IP had an IP of 10.12.1.250. The
problem is the WAN IP address, it needs to be configured? Can I just
use the WRT54g as a Wireless access point which will just forward
request to the firewall which should route traffic to the External
Network?

Does this make sense?