There's really very little to it. You turn it into a "router" by going into /etc/sysctl.conf and setting the parameter "net.ipv4.ip_forward = 1" and reboot (you can do it without rebooting, but let's not confuse things).
as for the firewall side, the standard firewall settings live in /etc/sysconfig/iptables. you can edit that file directly or use system-config-firewall
To most people routing also implicitly means natting, so IF that's the case, then you'd need to enable ip masquerading for nat. The best way for you to achieve this is probably to add a few lines that say:
iptables -A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -o eth0 -j MASQUERADE
(which means, in order of commands, traffic about to leave the system that is from the 192.168.122.0/24 subnet that is not going to that subnet and is going out of interface eth0 should be masqueraded to the address of the eth0 interface itself)
updating the subnets and interface appropriately. You can make this permanent by running "service iptables save"
That's really the basics of it. You should be comfortable with what you're doing before it goes into service though, there are so many ways you could possibly be at risk if you don't know what you're doing.