Most ethernet configuration is centralized in a single file, /etc/network/interfaces.
To assign the IP address 192.168.0.2 to the device eth0, with the typical netmask of 255.255.255.0. Your default gateway's IP address is 192.168.0.1. You would enter something like this into /etc/network/interfaces:
iface eth0 inet static
address 192.168.0.2 netmask 255.255.255.0 gateway 192.168.0.1