http://aboutdebian.com/network.htm
In a nutshell, edit the /etc/network/interfaces file and set it like
Code:
auto eth0
iface eth0 inet static
address 192.168.10.10
netmask 255.255.255.0
network 192.168.10.0
broadcast 192.168.10.255
gateway 192.168.10.1
Of course, change the address, network, broadcast, and gateway to what you need. Also, don't forget to edit /etc/resolv.conf to setup the DNS servers.