Use the reset button on the router. This is usually a pinhole button and you will need something like a paper clip or similar object to push the button. Hold it down for at least 15 seconds to reset the router. After the router reboots you will need to connect to it using an ethernet cable and going through the first time setup. The default admin user name and password are probably on a label on the bottom of the router. Log in and go through the setup. You will need to specify an SSID, encryption (use WPA2) and a password. You will then be able to connect by WiFi.