To set up networking the key file is /etc/network/interfaces. Examples of what to add to this file are included below, illustrating both DHCP for automatically obtaining network information from a server, and a manual setup.
# wajig restart networking
Note that to have networking started in this way you need to have auto eth0 in /etc/network/interfaces. Otherwise the start-up script /etc/init.d/networking will not start eth0. The start-up script calls ifup -a which only starts those interfaces marked as auto. You can still manually start the network with ifup eth0.