From what I've experienced they're completely random. From one appearing in Riverwood as soon as I left the trader's store, to seeing 2 almost back to back. Dragons are pretty easy if you have the Dragonrend shout and the Sanguine Rose weapon that casts a demon in Daedric armor to fight for you since he always takes all enemy aggro...Dragonrend, shoot Sanguine Rose to spawn demon, dragon lands, they fight, you jump in behind the dragon and destroy it. Easy.