If you have cable internet, then the device that connects to your cable outlet is the modem. Likewise, if you have DSL, the device that's connected to your phone jack is your DSL modem. You would want to have your ooma device connect to the output of the modem. Then, connect your router or computer to your ooma device. Ooma just needs the internet to work (ie the modem has to be up and running).
A PC is not required except maybe during initial setup.
If you have a modem / router combo box, you only have one box.
If you have a modem and a router, you have two boxes.