First, identify the BIG question. This one should cover the entire arc of the story. For instance:
Most action pictures: Will the good person prevail over the bad people?
Most romantic comedies: Will the two people end up together?
Most dramas and comedies: Will the protagonist get what he or she needs? (Quite often they start out going after what they want but then realize what they need is something else and go after that.) You would specify in your statement what the want or need or both are.
For the first two, most of the time the answer is “yes.” Even for the third, much of the time it’s “yes.” This means that in order to hold our interest you have to set and answer a bunch of other questions. However, being clear on that big question first makes sure you have a story spine. The next major question is:
What gets in the way? In other words, what obstacles will the protagonist encounter in the process of trying to vanquish the evil people, or win the girl/boy, or get what they need?
The logical follow-up is: How will the protagonist overcome these obstacles? (Or, if it’s a tragedy, how will he or she fail to overcome them?)
That gives you the basics of your plot. Next:
What’s fresh about this version of the story?
There are no new stories, only new ways of telling them. Ideally, you have come up with some interesting variations of the obstacles we’ve all seen. Intimately tied into this is:
Why should we care?
This gets us to characters. If we don’t like the potential lovers or the cop or your other protagonist or find them interesting, we’re not going to care about obstacles or solutions, either. Of course the more you know about your characters, the more ideas you’ll have for the kinds of obstacles and solutions. This means in a character-based story probably you will want to start with this question and then go on to the ones about obstacles and solutions.
Much of the time it pays to bounce back and forth, adjusting your answers as you figure out more about your characters and their challenges.
If you can answer these questions before you start writing, the odds are you have a sound foundation for your story. It may well change as you go along but you’ll have the comfort of a story skeleton to guide you.
(There is lots more writing guidance in my book, "Your Writing Coach" published by Nicholas Brealey and available from Amazon and other online and offline booksellers.)