A whole [story] is what has a beginning and middle and end. At the same time, a beginning is not itself necessarily after anything else and has naturally something else after it. As the below diagram suggests, your story needs a beginning, middle, and end. You should have an outline of the book before you even get started. It should be noted here that the first point is important because the climax is where all the story's emotional power is. If you write the climax first and are underwhelmed or bored, you will know that the overall story will not work.