Hi - You will need to have a router of some sort connected to your NTL modem (best via the ethernet cable). The choice is wireless or not wireless:
Not wireless: the router will have several ethernet ports, and using two (long?) cables, you connect each computer to the router. The computers will need ethernet cards (often comes as standard, if not they cost about £10). This probably means having cables running up the stairs - but you could also consider running them outside the house (like TV ariel cables) - or under floorboards etc.
Wireless: You will need a wireless router (though it connects by wire to the NTL modem!). This is more expensive, as are the wireless cards your computers will probably need (unless they are already wireless capable - as with centrino laptops). Two I have tried that work well are the US Robotics 8054 and the Apple Airport Extreme Base Station (the former is cheaper, the latter sexier - both work). Many others will work too, but I believe you don't want an ADSL modem - any stockist will tell you which ones can be used.
Hope that helps.