And when they say they want an author bio does that just mean for authors who've been published or have some sort of credentials? Not sure what they want with that.
If you have never been published before (like me!), I wouldn't worry too much about the author bio. I would just (quickly) list your writing background if you have one (degree, etc.), and I would instead concentrate on all the reasons why you are the only person who can possibly tell your story. (For instance, if you an expert in your subject matter--like a doctor-turned-writer-of-medical-thrillers--you definitely want to mention that.) Otherwise, I would simply keep your bio as short as possible, and let your writing speak for itself.
Best of luck!!