Whatever your writing style, if you’re serious about wanting to write something longer than a short story, it’s useful to create an outline, and take notes along the way.
An outline can be a brief list of bullet points describing your plot, goal of the story, main character (protagonist and antagonist), and obstacles to the MC’s goal.
i.e. A teenager wants to find his father but his mother forbids him because she knows his father is a bully, and deliberately hides any clues to the father’s location.
Free writing is a fun way to get a story started, particularly if you have an idea in your head and don’t want to lose it. Putting it into words is great…but often the idea dries up, or perhaps doesn’t coalesce into a longer story, which is where taking notes, and writing an outline (even after you’ve already started) can help gel that idea into a more solid plotline.
Using the example above, you could write your initial outline, in bullet points, then add to it. Annotate what kind of person the father is. Why did the mother split from him? Why is the teenager so desperate for answers? Why now? You can add more characters, too. A bff or a gang of bullies. A mean bus driver, or a gangster with a grudge. Go wild…but make sure it’s credible for your reader. Even if you’re dealing with aliens, there has to be a germ of “truth” to storyline.
i.e. it wouldn’t be likely that the mother in the example above is hiding the father’s whereabouts because she’s trying to hurt her son (unless she’s a sadistic psychopath - which is a whole other story). A more credible reason might be that she’s frightened of losing her son. Or that the father might hurt her son.
As you enlarge your story, take notes as you go, so you don’t forget your characters. Occasionally I’ll freewrite an entire novella, because the idea is so clear in my head that I don’t want to stop until it’s written down. I’ll then go back, once that first draft is done, and annotate the details, which also helps me to draw new inspiration.