For me, situation is story. I'm not a big believer in plot driver versus character driven stories. Stories I love are both. But in either case, I begin the story simply by putting what I hope is an interesting character into what I hope is an interesting situation. In other words, I give the character a problem to solve, a question to answer, or both. This is the basis of nearly all fiction.
Story is no more than how the character solves the problem, answers the question, or both. The situation, the problem or the question, forces the character to try to do something about it. This is where motivation comes in. The problem/question must be one that the character can't ignore, can't walk away from.
So he does something. Whatever he does has an effect, good of bad. He then does something else, reacts to the effect, which again has an effect. On and on.
As soon as I have the opening situation right, and drop the character into it, the rest of the story is easy. If it isn't easy, there's something seriously wrong with the situation, or with the character's motivation.