Presumably, from the heading, you know your router's IP address and that you type that into your browser's address bar. First link the computer to the router by ethernet cable.
Once you reach the router's setup login box, the Username is almost always admin.
Password varies according to brand -- sometimes admin (again) sometimes password. See the manual for your model of router. If the setup login password has been changed by the user you will need to press reset button on the router.
Once in the setup pages, go to wireless security and note down the passphrase -- or set a new one.