If you haven't changed the password from the default, look up your router model on google and find the user's manual (or google '[router model] default login').
If you have changed it and can't remember what you changed it to, you should probably just do a factory reset by holding down the tiny reset button on the back of the router for ~10 seconds (this would also be in the user's manual). Be warned that this will reset the router back to its original state, clearing any settings you have configured over time.