It begins with a smile, a name or an event. Your mind takes different aspects of things and extrapolates from them a story.
A fiction story has at it's base a field of experiences and thoughts gathered into one place and then expanded on. We gather those experiences & thoughts simply by living day by day. Sometimes you'll read something that interests you and you will save it in the file cabinet that is your mind. When you write you are opening one of those drawers and mixing the contents into something else.
This was taken in Niagra Falls on our recent trip there what do you see?
I see the remains of stone houses beside a rushing stream.
What is the story? Who lived there? What did they eat? How did they dress? What were the rules they lived by? Who was in charge? How far did their power extend? How were they destroyed? Who destroyed them? Why were they erased from history?
On and on the questions go. The answers to the questions will tell their story all you have to do is put it into some kind of order so it is readable.
Once you have an outline to work with the rest is basically just filling in the blanks. I am using a series of books from Amazon "Helping Writers Become Authors" The first book in the series is called Outlining Your Novel and the second is a workbook that goes with it.