Goto: Applications -> Accesories -> Terminal

Type: sudo ifconfig
(it'll ask for your password, just put it in)

See if under "eth0" there is "inet addrxx.xxx.xxx.xxx" where the x's will be numbers. DON'T PUT THOSE NUMBERS HERE. Just see if there are any there. if there are not, type "sudo dhclient eth0"

if everything is hooked up right and working, it'll pull an ip and you should be good to go. If not, it may start saying discover over and over.

A couple of other things to check. Look at the cable modem, there should be "ethernet" or "computer" or something like that label that has a light next to it, is it lit up? If you can't find that, read the brand and model number and give it to me. Also check and see if there's a "internet" label with a light next to it, make sure it's lit. That's all I can think of at the moment, but I'm sure I can come up with more if it's still not working