I assume you have a router connected to your ethernet port: if you've got a usb modem you've got a problem. The router is making the connection to your ISP, so you only have to set up the connection to the router.
See if you have a menu option to configure a network; if you don't, open a terminal and type the command "neat". The ethernet port eth0 will be listed as a device. Click on it and then on Edit. In the General tab, tick "Activate when the computer starts" (if you want to), "All users", and "Automatically obtain DNS". In the Hardware tab, tick "Bind to MAC". If no MAC is given, click Probe. Then click OK and confirm that you want the alterations saved. That's it!
If you want to switch the service on and off (the only secure network is the disconnected one ...), use the network control option in the menu, or the commands "ifup eth0" and "ifdown eth0".