Add a Wireless Access Point to the router (it connects the same as your current PCs do), then a WiFi card (USB or PCMCIA) to your laptop.
Note that there are several 'flavours' of WiFi at the moment - 802.11b is cheaper, slower and shorter range (but fine for web surfing, downloading etc). 802.11g is the newer standard, and is more expensive, faster and has a greater range. See click here for more info on WiFi.
Whatever you buy, look for a 'WiFi Certified' sticker, and choose a good brand name such as Netgear, D-link, Belkin etc.