The Northern Lights, as the name suggests, are mostly limited to the polar regions.
They occur most frequently in a 2500 km radius centered on the geomagnetic pole. This so-called auroral zone extends over northern Scandinavia, Iceland, Greenland and continues over northern Canada, Alaska and along the coast of Siberia.
Light-pollution, particularly city lights, can dramatically reduce visibility, so for optimum viewing one should try and get into the countryside.