and welcome to LQ!
I assume your set-up will be something like this?
INTERNET <------> eth0-Piii-eth1 <----> SWITCH
I'll also assume that the eth0-set-up as such is working
(the machine itself already has working internet connectivity)?
What you will be wanting to do is set up NATing for traffic
from eth1 to eth0 inside the machine. You do that using
iptables - if you're not confident with iptables, there's
tools to help you. One nice web-interface would be
where you can define all the requirements interactively,
and then download the script for your peruse.