Start off by writing about anything you want to write about, anything! This got me a head start. Start writing about your own life, like a biography, then add something different like:
When I was born...I had gained the power of fire!
Or:
When walking home from school one day, I bumped into a dragon!
Right when you come to a sentence you like, add more detail and add something different from life, you could make it anything you like. Add description and dialogue. Of course, the way I'm telling you to write is somewhat like Sci-Fi. [A.K.A. Science Fiction] But you can make it anyway you want! But make sure to make it original!