It is really very easy. In this example I will use the IP address 192.168.1.55, and my gateway will be 192.168.1.1.
Open the file /etc/network/interfaces:

sudo vim /etc/network/interfaces 

You should have someting like this:

# 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

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

Replace last two line with this:

# The primary network interface
auto eth0
iface eth0 inet static
        address 192.168.1.55
        netmask 255.255.255.0
        network 192.168.1.0
        broadcast 192.168.1.255
        gateway 192.168.1.1
 

Finally, restart the network:

sudo service networking stop && sudo service networking start