Edit /etc/networking/interfaces with nano change eth0 from dhcp to static as show below ( remember to use your own net ip addresses.

auto eth0
iface eth0 inet static
        address 192.168.1.20
        netmask 255.255.255.0
        network 192.168.1.0
        broadcast 192.168.1.255
        gateway 192.168.1.1