Yes, you need a router. Wired is easiest and cheapest if the Xbox is close enough to the cable modem. We've got a Netgear RP614 v3 and it works with the ntl modem. Just plug a network cable into the router and the Xbox.
If the Xbox is too far away for a cable you need wireless kit which is a bit dearer, especially the wireless adapter for the Xbox which is a ludicrous £50