Cannot get wireless connection signal at all - probably really stupid!

Hi – okay, this is probably going to send some of you tech geniuses screaming into the night…but, I’m really at a loss here and need some help. I need to setup a wireless router to work with my Dell laptop without having a 2nd system running at the same time. I have broadband Internet (Comcast), a Cisco wireless G broadband router and the laptop [running Windows XP]. I know how to setup the router using the instructions that came with the router: broadband cable from broadband modem into the router. Then there is the second cable that runs from the router to..??? If I plug this cable into the laptop then I cannot move the laptop and it’s no longer wireless. If I don’t plug the cable from the router into the laptop then nothing…no signal. I feel sure I’m missing something obvious, but I’ve spent the past three days struggling with this, trying different configurations and the only thing that works is plugging the laptop directly into the router – in which case, I may as well plug it directing into the broadband modem. I hope this makes sense to you – I’m aware that I may not be using the correct terms and also that this may be very obvious to someone more knowledgeable, but I really am lost and will greatly appreciate any help offered. Thanks!