How to switch modems?

firstly i am stupid…its just one of those things…i had tiscali but they went mental and deleted my contract so now i am with another provider who provided me with a wireless modem…i dont like that there’s no need its bigger and i live in a block of flats so people will be trying to log on all the time…when i plug my old modem in it doesn’t recognize the net,...how do i change it so i can use the old cable modem?