Writing your own book
If you're writing a story, think about interesting things in your life. You can think about ordinary things, and then alter the story a tiny bit so that it's believable. You could also choose a well-known story and alter it with your own ideas... Just make sure you give the original author and story credit.
Book report
If you're writing a book report or something, I would recommend Hansel and Gretel, or maybe a longer story, like Harry Potter or Percy Jackson, because they both start believable, and then arc into something weird and exciting. They are also both popular. I would choose Hansel and Gretel because it is also well-known.