Authors can be asked what got them to take the idea of being an author to the reality of a published book. My standard answer is that you have to stop thinking about it and start writing -- write anything related to the story.
What does "write anything" mean?
You may have ideas for scenes, a conversation, or even the ending -- the latter is good to know, but not necessary when you start a book. What stops many writers is seeing how to build from scene to scene to something cohesive.
Just write the scenes. They don't need to connect, you can change a character's name later, and you can reorder scenes. You can't do any of that until words go on the page.
The one thing you need be certain of is whose story you are telling. If you write mysteries, is it the sleuth's story or that of the murderer? You may have both points of view, but one is likely more prominent, and that determines a lot.
Do remember you aren't writing a screenplay in which the camera bounces from person to person. If you think you need ten points of view, you likely don't. It does depend on the story, but keep in mind that the more points of view you express the less there is to reveal over time. After all, the reader knows what most of the characters are thinking.
Here's a helpful article by Angela Ackerman on K.M. Weiland's blog. Ms. Ackerman tackles writing when you have no idea where to start. We've all been there and may stray back to that position from time to time.
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