If you visit Iceland during the months when it does get dark (August-March), you should check this website daily https://en.vedur.is/weather/forecasts/aurora/#type=total If the forecast predicts northern lights, and the sky is clear (both is very important, you can't see them if it's cloudy), make sure to look up that evening. It's a good idea to drive away from the light pollution in towns, if you're in Reykjavík you could drive to Grótta. You need to be patient, the lights come and go, so you need to be lucky.

Please note, you should never stop your car in the middle of the road, or anywhere on the road to look at the sky. Park somewhere that is meant for parking. Good luck!