You will need to edit the file /etc/network/interfaces and set it to start DHCP on boot. example file below.

Code:
# /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)

# The loopback interface
auto lo
iface lo inet loopback

# The first network card - this entry was created during the Debian installation
# (network, broadcast and gateway are optional)
auto eth0
iface eth0 inet dhcp
After editing the file you can /etc/init.d/networking restart from a root console window to re-start the network with the new settings. If this does not work could you post back as many details as possible about the network you are using.