Step One - The Initial Idea
Nothing is harder than the initial idea. There are so many things you could write about, so many different genre types to base your book and world within it around. Whether it is the mystical world of a fantasy, the stark and clinical corridors of a spaceship for a sci-fi, the carriages of a train for a murder mystery, or perhaps it is modern day in familiar surroundings, the world is your oyster.
All you have to make sure is that you pick something that you would enjoy reading, therefore diminishing the chance you will get bored with the subject matter and increasing the base of concept knowledge.
The only other thing that you have to worry about is that you aren’t plagiarising someone else’s creation without realising it. Sometimes ideas creep in without your knowledge and you have to take a step back and look at it and ask the question of have I seen/read this somewhere before.