what do I do to be able to get connected to bt pay as you go instead of bro?

I bought a laptop off my son, who had used it on broadband, I want to use BT pay as you go as I am a light user, but when I tried to install BT disc it comes up with ERROR you must have at least one modem configured on and connected to this system to connect to the internet" it will not let me go online at all, so how do I rectify this. Thanks Charles