If you want to maximise your chances of seeing the incredible northern lights, then a tour is a good idea. It can take some skills to get to the right area at the right time to spot them. So going with an experienced guide will help.
I recommend to join the small group tour by Icelandia. They have great guides and in case you don't see the lights, you can come back for free the day after. You will also get free admission to the Reykjavik Northern Lights Centre.
Icelandia also have big bus tours that are slightly cheaper and super jeep northern lights tours for the adventurous. My discount code "asasteinars" will give you 10% off on all of the options.
Remember to always dress warm and enjoy the show!