You will need a wireless router.
Some reccomended ones can be found
HERE that work on NTL. See the wireless ones.
You plug the cable modem into the wireless router with an ethernet cable. You will need to check if your PC downstairs has an Ethernet network card. If it has, simply plug an ethernet cable from that to the router, or alternatively buy a wireless card for the PC so that it too can connect wirelessly.
For the laptop, just make sure wireless is enabled (there may be a switch on the side of the laptop to switch on) and search for the wireless network signal in Windows.
Turn off everything before connecting new equipment, and remember to switch on the cable modem first, then the router, and then the computers.
Do remember to also secure your network once it is up and running.
More info
HERE