This is the original million-dollar question. The short answer is, whatever floats your boat: something that interests you will help you write something interesting. Variety, as they say, is the spice of life—so choose a broad theme to write around.
Jack Scott is an accidental author. His debut book became an Amazon best-seller as a direct result of successful marketing through his own website and social media. For more about Jack and his titles, check out jackscott.info.