This is a hard question for me. I first wrote a "book" back when I was five. It was a whole process and everything too. My parents bought a folder with one of those clip things in it, a lot of paper, and pencils and crayons. What I created with it was a berenstein bears fanfiction where I came up with the most original idea of, "What if they were actually bears." After that, I wrote more and more, but it was mostly childish fanfictions. There was some original works, but not any worth mentioning. I remember the first real fanfic I made was of me self-inserting myself into the pokemon world.
But what truly got me into writing was when I was around ten. Pokemon had a side game series known as the "Pokemon Mystery Dungeon Series." Well they came out with the final game of that series, and I didn't realize that until the gen 7 third game, which turned out to be two, was coming out. I was mad as hell, because there was a lot of story potential using the lore of the gen 7 legends. So I came up with my own outline for the game.
Then that outline changed. Suddenly, I turned every character into humans, I changed the magic system, started coming up ideas for sequels, and the next thing I know, I came up with a story wholly separate of the original concept. Then I actually began writing it. It was terrible, of course, because I hadn't ever written seriously before. But I loved every moment of writing it, and someday, I will publish it as its own thing.