If it is a Cable modem then you need to get two male connectors and one female/female connector, plug a legth of good quality satellite coax.
Put an F connector on each end of the coax. Now connect the coax to your existing cable with the Female/Female F connector.
If you don't feel up to making the connections without causing a short (Which will blow your modem -or worse!) then buy a made up piece of coax with a male F Connector on each end.
t, October 2008