As I see it, the problem you are going to have is to connect your *existing* PC as you will no longer be able to use USB directly. However, you can use a USB Network Adapter, either "wired" or "wireless".
What you have to do is to power everything off, including the Cable Modem. This is important so that the modem releases the association between your existing connection and the IP address it allocates. When rebooted, it will recognise a different connection and allocate a new IP address.
Connect the routers WAN (Internet) port to the Cable Modem's ethernet port and the new PC by ethernet cable to one of the router's LAN ports.
Power on the Cable Modem and wait for it to be fully booted, then power on the router and wait for it to be fully up and running. Finally, boot the PC.
If you now enter the router's IP address in to a web browser, you should be able to access its setup pages from where you can configure it. There should be very little to set up for internet access as you have a "direct" connection and get IP address etc from the ISP. The "wireless" settings may need a bit of configuring - give it an SSID (replace a default SSID), set it to connect to "802.11b" or "802.11g" or "mixed" as appropriate. Once you have it working, you can set up encryption to secure it.
Once you have everything working, you can connect wirelessly and remove the cable between PC and router.