Where to start when developing a story?

You can start generating a story from anything.
I’ve started scripts from characters, plot points, and endings, as well as geographical locations, themes, and titles.
But there’s an implied second question in where to start developing the story to script: "How do you connect the dots between plot points?” And even further: “How do I find a successful ending?”
So let me give you a short answer: Characters. Your characters should tell you where the plot should go. And in particular, your Protagonist and Nemesis.
Let’s consider 8 questions that I have come to accept as my process to develop my stories. If you can answer these questions, you should be on the path from your story’s beginning to ending.
1. Who is your Protagonist?
Determine which character is the most important one to your plot. Typically it is their goal, motivation and conflicts that establishes the backbone of the Plotline, the story in the External World (the realm of action and dialogue). Also it’s their transformation, going from a Beginning Emotion State to and Ending Emotion State (typified by four movements: Disunity — Deconstruction — Reconstruction — Unity) that dictates the arc of the Themeline, the story in the Internal World (the emotional / psychological realm).
2. What do they want?
This is the Protagonist’s External World goal, something they are typically conscious of. It is an end point that requires action and effort.
3. What do they need?
This is the Protagonist’s Internal World goal, something they are typically unaware of. It emerges from within their core essence, an authentic part of their ‘self’ that, as a result of the journey, emerges through the Deconstruction — Reconstruction process in Act Two.
Together the Protagonist’s want and need represent their goal. And this should steer you straight toward your story’s ending, especially when tied to the character arising in question #4.
4. Who’s keeping them from their goal?
This is the story’s Nemesis, the character (or characters) who provides opposition to the Protagonist. They have their own interest in the P’s goal, generally hoping for a different outcome. Typically the P and N will be joined in some sort of ‘final struggle’ in the story’s third act, giving the story a big event to build to.
Now answer the next four questions:
5. What is the story’s beginning?
If you understand where the Protagonist is in relation to their Ordinary World (the world they have lived in up to FADE IN), the nature of their want and need, and the problem this disjunction between want and need signifies, then you should be able to have a pretty good answer for this question.
6. What happens at the end of Act One?
You need to have a big plot point that transitions the Protagonist out of their Ordinary World and into the Extraordinary World of Act Two and beyond.
7. What happens at the end of Act Two?
A major setback for the Protagonist, an All Is Lost moment which causes the P to doubt their ability to go on.
8. What is the ending?
Again if you’ve figured out your Protagonist and Nemesis, and whatever Final Struggle they engage in, then you’ve pretty much got a cue on your story’s ending.
This is going to give you a start at developing the story. You can go further by thinking through the transformation journey the Protagonist takes in context to the conflicts along his/her way.