When someone I'm talking to learns I'm a writer, one of the most common things to happen next is the person saying they've always wanted to write, but they don't even know where to start. Since I'm in the process of starting a book, I thought that would be a good topic to address.
A book or story starts with the germ of an idea. There's something that makes you want to tell this story. It might be a character, a scene, a "what if" scenario, a concept, a message, a setting, a punchline, or anything else that lights up your brain. What you do next will depend on what kind of writer you are. There are people who will sit down and start writing at that point and figure out along the way what the story is. Since you don't know what kind of writer you are until you write, you can try that and see what happens. But don't get frustrated if you end up stuck or rambling. You'll have learned something about the way you work. Then you can try another method.
I generally find that I need to flesh out my germs of ideas before I can go anywhere with them. If a character came into my head, I need to figure out what kind of setting would be right for this person, what this person wants, and how that's affecting what's going on with this person. If it's a setting that sparks the idea, then I need to think of the kinds of people who might live there. And so forth. How you go about this kind of brainstorming depends on what works for you. I generally have a lot of idea scraps floating around in my head, and I take them out to play with every so often, usually while I'm taking a walk, washing dishes, driving cross-country, or lying in bed as I fall asleep or wake up. I don't start doing serious development work until the idea fragments feel developed enough that I can see a story taking shape. There are times when this can take years, or it might all come together with a quick snap.
For more formal brainstorming, you can try techniques like mind mapping, where you start with your idea and branch off related ideas. Some people do physical collages to uncover more details about the idea. Or you could just make lists of things that occur to you. Once you have a little more detail, enough for the story to have a general shape, you might see that you need to do some research on the setting, professions, time period, technology, or whatever other elements are at work in your story. That will give you more ideas.
From there, you might try starting to write again, but if you're still stuck, don't worry. Some people need to plan and plot. Do detailed development work on the key characters you've identified and start outlining a plot. Some people go all the way to storyboarding individual scenes. If it's your first attempt at writing a story, it's mostly about finding the way you work and learning just what it takes for you to be able to write. There's no better or worse way to write. You're only judged on the final product, and no one needs to know how you got there. You don't get bonus points for just sitting down and being able to whip out a novel based on a vague idea, nor do you get bonus points for doing lots of preparation work. The bonus points come in having a great story that readers enjoy.
So how do you start? Just start. Try writing a first line. You can always change it. If you can't think of a first line or an opening scene, step back and do some planning and plotting and then try again. Keep at it until you have a completed story. Then on your next attempt, you'll have a better sense of what works for you and what steps you need to take to be ready to write.