Everybody writes plots differently, you've really got to find a method that works best for your mind. Usually plots start with world level concepts for me, then I start asking questions and weave my plot from the answers I come up with.
Let's look at Star Wars: It's a sci-fi story. Naturally, a sci fi story would have spaceships. How big are these ships? Can a spaceship be as big as a planet? What if it could destroy other planets? What kind of people would build a weapon like that? How did this happen? What kind of political changes would result? Who would try to stop them?
You can see how asking questions about basic conceptual elements would build the world up to a point where you can start pinning down events and start imagining vague characters in that world. Then those answers lead to more questions, and so on and so forth, until you've just spun up your own universe. Then you start fleshing out some characters with goals and personalities. Most of my characters appear out of necessity. As I'm laying out a scene I'll think about what kind of environment it is, and who would be there. What would they be doing? Why are they there? How would this relate to the plot or the world building? One of my favorite questions as I'm laying out plot developments (and especially action scenes) is how can I make this worse? That one always yields some interesting fruit. I've obviously been mainly talking about fiction world building here, but these concepts can apply to building a plot in a real world setting as well, you just start out with a lot more of your questions answered for you.
As I'm answering these questions I start to ask more philosophical questions like what do I want to say with this story? What should my overall tone be like? Why do I want to write this? And most importantly: What's my theme?
These questions guide both the events in the world, and how your characters interact with the world and are built by it. The theme is particularly important, but also can be particularly difficult. Your theme needs to be clear enough to where you can boil it down to at most a couple words. For example, the theme of Heaven Hunters is guilt. You can see this woven into plot developments, character developments, even the general world building is wrapped around this one word. I had to do a lot of writing before I got to a place where I could define what I was writing about so succinctly. Once you have your theme though, it becomes a guiding light for every decision you make. Every character, every plot development, every world event builds, or interacts with, or explores that one idea, and that's how you write a clear story.
Then there comes organization. There are various methods of organizing your plot/character arcs, the three act structure is the most common I think, and for good reason. The hero's journey might be a good thing to do some research on and study it's application.
As you write more about your story and it's characters, you'll find yourself gaining traction with it. This just takes time and work. It took me three months of obsession and constant rewrites before Heaven Hunters became even remotely like what it is now. Most importantly, be patient with yourself, and enjoy it!
