Hi Larry. To add POE cameras to what you have, you would need to purchase a POE capable switch.
Such as this:
Amazon.com: TP-Link 8-Port Gigabit Ethernet PoE Rackmount Switch with 8-PoE+ Ports (TL-SG1008PE): Computers & Accessories
This switch would provide enough power, that if you wanted to add a PTZ camera, this can handle it.
As far as hooking it all up, you would take one ethernet cable, plug it into one port on the new switch.
Then plug the other end into your router port that you have empty. That gets the switch and the router talking to each other.
Then when you install cameras that are POE powered, you would plug the ethernet cable into the camera, then the other end of that cable would plug into the new switch.
Thats pretty much it. The switch provides the power to the cam, and allows you to connect to the cam with your Viewing device.