A common problem that occurs when writing a book or screenplay is that it loses focus. There are interesting subplots, and interesting side journeys, and after awhile it's hard to know what to pay attention to. Yes, ever since Tolkien published Lord of the Rings I know many writers want to do this sort of complex world-building, but frankly I haven't seen too many of these epics actually being published. Heck, even Peter Jackson found he had to cut A LOT of Tolkien's material in order to get a comprehensible story line -- and his movie masterpiece trilogy is still 9 + hours long.
It's worse if you're not even trying to branch your story out in 32,853,02 directions.
I'd like to propose a few easy questions for you to answer about your story, that should be able to focus you in to get your story started with minimal trauma. If you can answer these questions, you've got the story's spine. For every event or character that you want to add, simply ask yourself if it's consistent with what you've already laid out here. If it is, go for it. If not, get rid of it. This includes things like subplots: the subplot should either be adding a component that is necessary for the story usually at the finish, or following a mirror character where the character wants the same thing as the protagonist, but answers the question in a different way.
Ready? Here are a few questions to help you get at the essence of your story:
1. Who is your MAIN CHARACTER?
2. What external problem does your main character want to solve in the story? This is his OUTER GOAL. For example, he may want to win the big football game, or make a million dollars, or find a girlfriend.
3. Who or what is the chief OBSTACLE to your protagonist's achieving his outer goal?
4. What horrible things will happen if the protagonist cannot achieve his outer goal? This is the STAKES of your story.
5. What is your main character's HIDDEN NEED? This is a lack within your main character that he must solve before he can be happy. For example, he may need to forgive someone, or he may need to become courageous, or he may need to learn not to be selfish.
5. In one sentence, describe what your story is about.
These questions may be easy, or may take some thought. If you're having trouble, simply list, say, 10 or 20 stupid answers to the question. Then just pick one of these answers and see if you can fit it in; if you can't, choose another. Free-write your ideas so that you can tell a quick outline of your story in a paragraph or so. Figure out the captivating kernel of your story, whether character, plot twist, or something else.
Once you've got the basic direction of your story, you'll find it's much easier to start planning or writing.