It seems so simple that most new novelists don't think it can be the key to a story, but it is. It defines the spine of the story, the direction and the source of the conflict. Ready?
Find out what it is your protagonist passionately wants or desperately needs.
Start him/her on the path to attaining it and then put something almost insurmountable in the path.
This is all there is to it. If you do this, you have your story engine. If you don't, you will keep circling the parking lot looking for the on ramp.