Writing a book

I was thinking about writing a book ever since i wrote a story about a monster eating Sutter's mill because that came to me so quick and it was a pretty good story. My dad told me i was going to write a story. He wasn't telling me too he was just saying that he knew i would write a story. The problem is i don't know what to write about. I'm sure if i thought of something i could write about it because i could write about anything. I could write about just a plain old piece of grass. That would be easy. So give me some ideas.