Honesty. You can't lie to the reader anymore than you can lie to yourself. If a story doesn't feel real to you, it won't feel real to the readers. That's not to say a story has to be grounded in reality. Not at all, but it has to be real where it counts. Look at Harry Potter and how millions of people connect to the series. I left 11 behind many years ago, but I'm still waiting for my letter.
I can't remember a time when I was not making up stories in my head. For as long as I can remember I escaped into books and my imagination. Officially, I have been writing for three years.