Using a static IP address
As root (or via sudo) edit @/etc/network/interfaces@. You will have something like this:
auto eth0 allow-hotplug eth0 iface eth0 inet dhcp
Comment these lines and add the following ones (replace with your values):
auto eth0 iface eth0 inet static address 192.0.2.7 netmask 255.255.255.0 gateway 192.0.2.254
Tweak @/etc/resolv.conf@ if you need to adjust your DNS settings.