When I first began writing every how-to book suggested that a writer begin writing what they know. But while this advice might be great to jumpstart a writing career, unless you have a really unusual life, you may run out of things you "know" pretty early in the game. And sometimes what you "know" can be an obstacle to creating good fiction.
When we know a subject very well, we're sometimes too close to it to fictionalize. For instance I love movies or books where one character is a writer and her editor actually comes to her house to help her write or just to visit. I've been writing for almost twenty years and I've never had one of my almost a dozen editors ever come for a visit or to help me write. If you don't live in New York or L.A., chances are rare that you'll see an editor anywhere except at a writer's conference or if you go there to meet them. But I digress -- I couldn't write that scene because I know too much about writing and yet for the purposes of many fictional stories and movies, it's kind of cool to have an editor visit the writer.
Now, you might be saying but if it wouldn't really happen, then you're wrong in writing it -- no, that's why it's called fiction. As long as it can happen within the realm of possibility, you're cool.
So once you get past the "write what you know" advice, my advice is to write what you DON'T KNOW but WOULD LIKE TO LEARN. Researching has been one of the most enjoyable parts of my writing career. I have learned how to sail, grow orchids, ride horses, win the Kentucky Derby, cure amnesia, steal million-dollar paintings, catch thieves who steal million dollar paintings, find famous missing Russian orphans, solve cold case murders, find long lost family members, sew quilts, move up in the police department, decipher psychic visions, change my identity and find lost shipwrecks. And I rarely left the house.
Writing what I don't know engages my curiosity, gets my energy going, makes me excited to create a world where I'm going to share some of what I've learned, and I do mean "some". Never overburden your fictional story with too much research. Carefully pick and choose only those points that make your story better. Always remember that you're telling a story, not writing a how-to book.
So if you think you don't know anything worth writing a book about -- think again! The world is waiting. What have you always wanted to learn? Where have you always wanted to go? Who have you always want to be? As a writer, you can do it all!