wireless: just enable Airport and connect to your network as u were on PC. And makesure u add the iBook to you router if u using IP filtering.
cable:
1) Makesure ur router is using DHCP
2) Connect the network cable
3) Go into the System Preferences (i presume u are on OS X)
4) Click Network
5) Click on Built-in Ethernet
6) Then click on "Configure"
7) On TCP/IP choose DHCP, if everything is fine you should see the IP address after a while. If not go to nex tab & uncheck "Connect using PPPoE".
8) You should be able to online now if nothing is broken.

try restart ur router.