Normally just connect an Ethernet cable from your computer to the router (or connect wireless if you can) then open a web browser such as Internet Explorer and navigate to the IP address.
If you are unsure of the IP, it is normally written on the bottom or the back of the router along with the wireless key, SSID and login details.
Alternatively you can connect to the router then open a command prompt and type: ipconfig /all
This will list some network information. In a home network the IP of your default gateway will almost always be your router.