Aurora Buddy App
This Android app is free and available on the Google Play store here: Aurora Buddy
Basically, with this app you set your location and then the app will send you an alert on the days where you can view the northern lights.
More about Aurora from NASA
If you don't have an Android phone, or you just want to look at an aurora now, check out this cool video from NASA. Below the video is a paragraph explaining how these form. There's also this video, which shows how they happen.