First to get networking going. MAke sure you have the correct module for you nic in /etc/modules, then edit the interfaces file (nano -w /etc/network/interfaces)

And for dhcp you will want this
auto eth0
iface eth0 inet dhcp

for static
auto eth0
iface eth0 inet static
address X.X.X.X
netmask X.X.X.X
gateway X.X.X.X

the save and exit, then as root do ifdown eth0 && ifup eth0