See introduction of DHCP_Server.
There are different DHCP client packages around. The default for Debian seems to be dhcp-client. It is probably installed already, as you can check within aptitude. But to be sure you can run as root
aptitude install dhcp-client
For a basic configuration you have to edit the file /etc/network/interfaces, which defines the interfaces of your computer. If you want to use eth0 as the interface to be configured through DHCP during boot up, add/edit the eth0-entry
auto eth0 iface eth0 inet dhcp
The different keywords have the following meaning:
auto: the interface should be configured during boot time.
inet: interface uses TCP/IP networking.
dhcp: the interface can be configured through DHCP.
That is all.
To change the system to use a static IP address (not dhcp), change the /etc/network/interfaces file to something like:
iface eth0 inet static address 192.168.12.2 netmask 255.255.255.0 gateway 192.168.12.254