There should be a file called interfaces, net.eth0 or ifcfg-etho (to find it do find / -name eth|grep etc; it should be in etc) that you edit to assign static IPs. It's location can vary based on your distribution.
A basic configuration (at least in Debian/Ubuntu) would look something like this :
iface lo inet loopback
iface eth0 inet static
Where address is the IP address, netmask is the subnet mask and gateway is the IP of your router (only if you are using one)
You then edit /etc/resolv.conf to set your DNS servers.
Note that this process may vary depending on your distribution.