You might need to change your /etc/resolv.conf, you will have to add a line like:
(where the ip is the ip of the router or server). You will need to edit /etc/network/interfaces aswell. It should have these lines in it:
iface eth0 inet dhcp
Then restart the network:
ifdown eth0; ifup eth0
That is how I normally restart the network, but there might be another way. Hope this helps,
edit: Just thought, don't you need to install some DHCP client programs? And if the network starts at boot, then it should use DHCP at boot aswell.