A:
Assuming that you have a wired Internet connection to your computer, just unplug the Ethernet/Internet cable from the back of your computer and plug it into the jack on the wi-fi router that's labelled "Uplink" or "Modem" or "Internet". Then connect a new cable from your computer to any one of the other jacks on the wi-fi router.
If you only have wi-fi access, you need a different device: a wi-fi acces point. That kind of device will allow you to use the wi-fi signal to connect devices that need a wired connection.