Assuming there is nothing wrong with the onboard NIC, DHCP server, cable, switch etc, the following should help.
First, find out the interface name of your onboard NIC. Usually this is eth-something. Run the following command soon after boot:
You should see a couple of entries like the following:
One will be your onboard, the other the PCI card.
eth0: RTL8168c/8111c at 0xf810a000, 00:11:22:33:44:55, XID 1c4000c0 IRQ 27
eth1: RTL8168c/8111c at 0xf810a000, 66:77:88:99:AA:BB, XID 1c4000c0 IRQ 28
You will need to be root to do the rest:
Now edit /etc/network/interfaces. From what you have said I am guessing it will only have the following:
Add the following at the end of the file (substituting eth0 for the interface name of your onboard NIC):
# The loopback network interface
iface lo inet loopback
Make sure the NIC is plugged into the switch, then bring it up with this command:
iface eth0 inet dhcp
It should then get an IP from DHCP.