First, you have an idea for a story. But that's not really worth much. Some people will just start writing with that, and if you have enough experience maybe that will lead to something structured. But for most people that will lead to a rambling mess. If you just want to write, that's fine. But if you're goal is a book, something that other people can follow and enjoy, and maybe be willing to pay to read, then you need to have structure. There are different levels of planning that will allow you to write to a structure and it's important to know what you need.
For me, it works like this. I have an idea. That can mean a lot of different things. Sometimes it's a character. Sometimes it's a scene. Sometimes it's a deep philosophical concept that tugs at the mind. Though I often find my stories come from a reaction to someone else's story. I read something and think that it would be more interesting if approached from a different viewpoint. So I'll start with that viewpoint and see where that leads me. For instance, there's a classic fantasy convention of the kitchen boy becomes king (King Arthur). The idea is that you start with a character who is unsure and unlikely and over the course of the book, or trilogy, or Robert Jorden never-ending continuum, they develop. Characters that develop are interesting. And most readers can identify with a character who starts as an everyday person. If you start with a king, someone who has the power and keeps it, someone who has everything figured out and doesn't need to change, well, what's interesting about that? That's what's interesting to me: how to make a hero interesting when he starts as a hero. Based on that simple concept I have an entire fantasy epic worked out (though not the time and patience to develop it at the moment). But it's the idea that starts it.
But how do you work it out from there? For me, it's an outline that keeps getting filled in. I'll outline the general concept of the book. Two or three sentences that sum it up. Then I'll think about it in terms of acts. There's a reason most plays have three acts and it's a solid structure to build around (though not necessary). Once I have the acts, I'll try to break it into parts. The number of parts in an act will vary, but the idea is to have a sense of how the act will flow. From there I'll break it into chapters. Each chapter will get a paragraph summation. So at that point I will have a chapter by chapter outline of the novel, but built from the outside in. I've tried starting with the chapters but normally end up missing something (you only see that when looking at the whole). And once I have the full outline I summarize it. For me it's invaluable to have a simple one sentence per chapter outline that really shows me the whole story at a quick glance.
And that's what I'll write from - my chapter outline. It turns a long, complicated story into simple pieces. Pieces that I can accomplish, that don't overwhelm me. By focusing a piece at a time I can dig in to each one and lose myself in the moment. But by planning the whole I know my piece will be part of something much more complete. A lot of the unpublished stories I see and read have fine writing, but they roll and ramble through a series of interconnect events that somehow doesn't satisfy. They drag in places and rush through others, ideas pop up and disappear, there's no sense of completeness that readers ultimately want (whether they know it or not).
Of course, many great writers and great stories throw this all out the window. Some people just start writing and greatness comes out. But greatness is something different. I don't have the skill to aspire to greatness (not yet). I simply want to create a good story that others will enjoy. Maybe I'll stumble upon greatness one day, but I don't think you can plan for it. So I plan for goodness and go from there.