Assuming that your NTL broadband is over your telephone line, you'd need to replace the modem with an ADSL modem router (this is a separate box). You'll also need a wireless adapter (USB is easiest) to plug into the computer. You can often buy a package that includes the router and the USB wireless adpater in one box. The kit should come with instructions.
The router should also support wired connections (using ethernet Cat 5 cables).
Note that it is recommended to have at least one PC set-up with a wired connection as wireless is not 100% reliable and if you have problems with the wireless you won't be able to access the router to sort things out (as it is the router that connects to your broadband service and holds all the login information etc.). So if you only have one PC, it is best to stick with wired (or at least set it up so that you can use both wired and wireless).
I would guess that it is pretty similar if your braodband is over cable, but I don't knwo the details.