Ensure that the router is powered on (lights on) and the cable between the computer and the router is clicked in at both ends.
Make sure your computer is plugged into one of the LAN ports, not the WAN port. One of the LAN lights on the front should be lit (flashing).
Make sure your computer is set up to get an IP address automatically. On Windows, you can check this by going to the Control Panel, then Networking or Network and Sharing Center, selectChange adapter settings (in Windows Vista and 7), right-click on Local Area Connection and select Properties. Select TCP/IP or Internet Protocol Version 4 and click Properties. Make sure Obtain an IP address automatically is checked.
On a Mac, go to System Preferences and select Network, then Ethernet and make sure Using DHCP is selected.