Recently, I received a message asking for how to develop a story's structure as well as its base or starting point. I'm going to say first that there are multiple methods for how to create either, and the method I use is only one of those. So if it doesn't work for you, don't think that it's the end of the road; it might simply be that another method works better for your writing style. And if this method works for you, that's great, and I am happy to have helped!
For me, I've never liked developing the entire structure of the story in one go; I prefer creating the beginning and ending first. I almost always write the middle part of the story as I go with a few key points to follow here and there. I feel like this provides me with more flexibility as I craft the plot since I always find that something changes along the way. With this in mind, it's totally fine if your ending of your book changes from what you originally thought. It happens, and sometimes changes need to be made that alter the ending for the story to make logical sense.
Due to this, I'm mainly going to give suggestions on the beginning of the story since the rest, ultimately, will depend on how the remaining chapters progress after the opening ones. For me, I like to select the setting or environment first. Will it be in a mansion, forest, mountain range, etc? Keep in mind, your initial setting doesn't have to stay the same for the whole story; the characters can move locations depending on where you want to take the plot. I generally know its the right starting environment for the story when I just get a click in my head; the feeling of, "Yeah, that's the one!" This may take some time, and it usually takes me about a month in advance to pick one that I really like for my next story.
After the setting, I decide upon my protagonist's personality. Appearance isn't vital unless I'm publishing on Amazon since the books are reader inserts initially. But, her personality probably will change and develop as she interacts with the love interest, who usually is the antagonist. The same goes for the antagonist. I also create a few minor and possibly major characters too that will be in the first few chapters; their personalities might develop depending on how large their role in the story is. Once this is done, I start writing. The next post, I'll probably cover how to write and work on that opening paragraph of the first chapter, and how to make it a hook.
Overall, though, my advice is don't freak out if your story structure is very loose and changes a lot over the course of the plot. It allows for flexibility, and it's often given me the ability to write some of my best chapters or moments in my books. Anyway, I hope this helps!