Here's what I'd follow:
First up, do an lsmod to ensure your module for your ethernet is loaded.
Second, determine how you obtain an IP, if it's Dynamically (I am assuming since you are trying to use DHCP) then you will want to find out at what point in your startup scripts (if at all) you have networking setup. Look there and see if it's trying to setup a static IP or something else you aren't using.
From there I'd manually run:
This is for clients, you. If you were running a dhcp server, you'd run dhcpd to startup the dchp daemon for others to grab an IP from
From there, look at any other hardware you may have. If you've got a router do you have it set to work with DHCP for the boxes connected to it?