Edit /etc/network/interfaces and adjust it to your needs (in this example setup I will use the IP address 192.168.178.50 for eth0):

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface 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 interface
auto eth0
iface eth0 inet static
        address 192.168.178.50
        netmask 255.255.255.0
        network 192.168.178.0
        broadcast 192.168.178.255
        gateway 192.168.178.1
        #optional
        #dns-nameserver 102.168.178.1

You can also edit /etc/resolv.conf to configure a dns-server to use:

nameserver 192.168.178.1

Edit

/etc/network/interfaces 

and adjust it to your needs (in this example setup I will use the IP address 192.168.0.100):

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface 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 interface
auto eth0
iface eth0 inet static
        address 192.168.0.100
        netmask 255.255.255.0
        network 192.168.0.0
        broadcast 192.168.0.255
        gateway 192.168.0.1