Start off by editing the file /etc/network/interfaces with your favourite text editor.  You will need to have root access to modify this file, so for example use the following command:

sudo pico /etc/networking/interfaces

Now tweak the file to your needs, this example sets the IP address to 192.168.1.123 on eth0.  I suggest running the command ifconfig beforehand and reviewing its output to determine which ethernet or wifi adapter you’ll be using (ie: eth0) and your network (ie: 192.168.1.x).

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interfaceauto loiface lo inet loopback
# This is a list of hotpluggable network interfaces.
# They will be activated automatically by the hotplug subsystem.mapping hotplug
script grep
map eth0

# The primary network interfaceauto eth0

# Enable for DHCP
#iface eth0 inet dhcp

# Enable for Static IP
iface eth0 inet static
address 192.168.1.123
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1

Now restart your networking interfaces using the following command:

sudo /etc/init.d/networking restart