Start by writing down anything that comes into your head, particularly your dreams. Get into the habit of doing it, just in note form. Not sure whether you are able to carry your iPad around with you, but if you can, use Notes on that, as an example.
When you read through your notes, make sure to write anything down that comes into your head while you are reading.
That advice is from Neil Peart's song writing method.
Read a book every day, and write as much as you can, not necessarily with the intention of writing a complete novel. Use your writing as part of your notes.
From Isaac Asimov, on How to write Science Fiction.
Start with an idea. You only need half a page to convey the entire story. Make sure your half a page has a beginning, a middle and an end. Develop your characters to the point where you feel you know them, care about them and understand what they would do in any situation. Then flesh out your story with the details. If your idea is solid and your characters well developed, they will write the story for you.
That's more or less what Arthur C Clarke wrote in the back of one of his anthologies.
That should get you going. By the way, I've never written a book, so don't take my word for it.