Modem port to Router WAN port, your PC goes into a router LAN port
your router will have a defaul IP for the internal network, such as 192.168.1.1, and it will give your PC an internal lan ip in it's DHCP range
If you connect the router WAN to the modem port and it doesn't work, then do this first:
1. Power off both devices, unplug the coax from the modem
2. If you modem is also providing voice, sometimes it will have a battery, pull that as well. Leave everything powered off fully for at least 5 minutes
3. Plug in coax and modem power(insert battery first if removed before) only, wait for modem lights to come to a ready state
4. ensure ethernet from modem is in the routers WAN port, then power up the router
5. wait for router to come ready, ensure the PC is plugged into a LAN port on the router, and boot the PC
6. If windows, open a command prompt and type: ipconfig /all
In there, look for your IP, and look for the default gateway, the default gateway is your router's IP
Put that IP into a web browser and you should hit the router's config page, but you'll a login for it.