What you're asking is very easy to do. The file that you need to edit is /etc/network/interfaces.
If you're currently set to accept a DHCP address, you should change the line that reads
iface eth0 inet dhcp
iface eth0 inet static
and then add any of these lines as appropriate to your system:
If it is an internal network, and all you need is an IP, then I think you can get away with just an address. Most times you will need at least an address, gateway, and subnet to get out on the internet. The network and broadcast ear easy to figure out, they are usually the lowest and highest addresses in your subnet.
Once all that is changed to how you like it, do
and then follow with
and you'll have the new address activated.