It can be an epic vision or something as small as a quarrel in an Inn. Make sure that you don't have a set path to get to where you the story to go, because the players will dictate the path the plot takes. Do, however, create plot points to hinge the story on. Plan for most contingencies and be ready to change things on the fly. Make your NPCs interesting and different from each other.

It is honestly about being organized, flexible, creative and needing to create a fun experience for other people.

Of course, with that being said, be mean to them sometimes