Ideas for books come from a zillion places.
Sometimes a character just pops into your mind and refuses to leave. Their voice must be heard and you build your story around them.
Sometimes you'll dream a scene, or an entire plot, and fashion your book from that.
Sometimes a current event on the news or something in your personal life will spark an idea. Or even reading a poorly written book.
But what if all you know is you want to write a book? Where do you start? How do you pick a genre or find a plot?
I recommend your first stop is your own bookshelf (or your library history). What do you read? What do you love? Divide your books into genres and count how many you have in each. The genre with the most books is the one you should be writing in.
Then I recommend googling that genre, learning about it. What are the best sellers? What are the typical story lines and conventions for that genre? I feel comfortable guaranteeing that somewhere in your study of the genre, you'll stumble upon a spark that will start your book.