ubuntu

This is an instant 2-command process.

Code:

echo 1 > /proc/sys/net/ipv4/ip_forward

Will immediately begin forwarding IP's (routing). Editing /etc/sysctl.conf you can turn it on permanently.

Since your internal IP's are probably not valid on the internet you will need to do IP masquerading too. This is accomplished with IPTABLES.
Code:

sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE