You need a Router.
Dont just buy a Hub. Note a Hub will split a single cable into a number of sockets, 4 or 8 for example, but only within the network NOT at the point of entry.
A Hub (or switch, which is similar) is NOT the same as a Router.
For example I have a Router at the point the cable comes into the house.
Our family PC is plugged into that router.
I also have a single network cable plugged into the router which goes up to my office on the first floor.
In my office I have a Hub in which I can connect 2 or more computers.
Note there are two main types of routers, on for "phone line" customers (ADSL) and one for "Cable" customers (Virgin, NTL, Telewest etc).
Make sure you buy the correct one.