will need an adapter, unless it's close enough to the router to connect the two via an ethernet cable. If it is close, that's the best thing to do - just plug the cable into any of the LAN ports on the back of the router (normally there are four) and into the computer's conventional network adapter - there will be a LAN port at the back.
You can certainly use a USB adapter for the laptop, although a PCMCIA card type will run a little faster. Having said that, in normal use you aren't likely to notice much difference. Install the USB adapter's software before you plug it in, by the way.