install firestarter to manage the port forwarding and NAT setup
yum install firestarter
make sure that works first with two nics to share the internet connection with your internal lan. Follow the setup wizard to configure internet connection sharing with the dual nic.
Next configure squid to listen on port 80 on the internal lan port. This is called a "Transparent proxy", give google a quick run for "transparent squid proxy" for a few good articles on the setup.
I also recommend using webmin http://www.webmin.com/
as a way to set up and configure squid for transparent proxying without reading every line of the squid.conf file. It has a button in the squid configuration area that automatically configures the firewall and squid together