Try creating an outline, and then using that as a reference to write the story.
One of the first books I wrote as a teenager, I actually did this. I just wrote down an outline first; pencil on paper. Not chapter by chapter, but event by event. When a major event came to mind, I would use the top-level outline numbering and indent level, and then drill down into what would transpire. It was a great exercise to help train my mind how to write books.
As I wrote the story, perhaps a year later, I remember finding it easier to write about one event at a time. It was more like writing a series of short stories. Of course, I changed things at whim. Through several rounds of rewrites, the book did eventually come together.
I hate to say it, but if you're writing a story, don't be afraid to just outright use a formula. I don't do this any more, but have certainly used it as a mechanism to help get ideas out when I was younger. Just think about how you think the basic story should flow, and then fill in the blanks with passes of ever-increasing detail, sketchy outline, detailed outline, rough draft of events, piece together into short stories, and then craft a draft novel (or series, depending on the amount of stuff you're covering).
- Opening scene
- Plot development 1
- Plot development 2
- Ending climax
Looking back, this may not have been the easiest way to create a book, but it was a way that worked for me, given being wont to overfocus on details and then actually lose focus on the story as a whole.
I'd say that anything you do that helps you get out the stories you have inside is a good practice. Your talents will develop better as you write than if you're thinking about writing, but not actually doing it :)