Ideally you would install a PCI card in your PC and a PCMCIA wireless card adapter in your laptop (unless it has WiFi built in).
If you find opening your computer's case a daunting prospect don't worry - an external wireless USB adapter will be fine; just remember to install the software before you plug in the adapter.
Alternatively, if your PC is near the router you could connect them together physically, using the Ethernet cable that will come with your router. Connect the cable to the computer's existing LAN port, and to any of the LAN ports on the back of the router. You'll need to do this anyway, to intially configure the router, so you could simply leave the cable connected permanently and just run the laptop wirelessly.