Every story should have a beginning, a middle, and an end in order to be complete. You learned that in school, right? 
Of course, like everything else, when you come to write the story it's not quite so simple as it sounds to have that Beginning, Middle and End.
THE BEGINNING: Where should your story begin? New writers often make the mistake of starting long before the real story begins, in an attempt to get their reader orientated. But think about this for a moment. If you were to tell someone about your day, how would you start? Would you begin by describing how you woke up, got up, ate, dressed and went about your daily work, right down to the details of dental flossing and crisping the bacon just the way you like it?
Of course not - you'd start at a point where something interesting, something unusual, happens. The reader can assume that your protagonist (main character) got out of bed that morning and got his or her day going something like the way the rest of us do - except, of course, unless the unusual event starts as he wakes up, like the bedroom ceiling collapsing on him….
So, consider where your story actually starts. Let's make up an example. Suppose your hero was orphaned when his parents died in a car wreck when he was only five, and he had a difficult time being raised in foster homes, or maybe he had a good life with loving adoptive parents. But now he's 31 and is just opening his mail on a sunny Tuesday morning. There is a letter from someone purporting to be an old friend of his long-dead father, claiming that Dad never died in a car smash but has been living in Las Vegas under an assumed name all these years.
Where does the story start? With the opening of the letter, which is the beginning of the hero's quest to find out the truth. The details of his childhood are all interesting backstory - in fact, you could perhaps have a prologue where the car accident takes place, work in something a little fishy, and show how he came to believe he was orphaned. Details of his life after that - abusive or loving home - can be worked into the story in little pieces as we go along, and they will add color and richness to his character.
Each beginning is special - you choose what we call a 'hook' or particularly exciting statement to open with. We'll be discussing this later.
THE MIDDLE: the middle is the rock on which many a good novel founders. This is where you may run out of steam, unless you've done some good planning beforehand. I always suggest that writers be careful with the amount of planning they do, however - if you do too much outlining, it can feel as if you've already written the story and you lose the excitement which is so important in carrying you through to the end.
But the main thing that gets you through the middle is applying the beginning, middle and end structure to every chapter. Each chapter is a scene, or perhaps more than one scene depending on length and each scene has a beginning, a middle, and an end in action. Therefore, instead of 250 - 350 pages with one beginning, one middle and one end, you are actually going to write your novel with possibly ten or 20 beginnings, middles and ends - one each per chapter!
ENDS: Often, when the story idea strikes, you'll also have some idea how it ends, or how you want it to end - sometimes when you're writing, what you want and what actually happens can be different. However, it works much better if you know what you want the ending to be. This way you can shape your story so that the it moves organically towards that ending. Endings should always be satisfying, and if you have a little surprise in there, all the better. But you must tie up all the loose ends and clues and hints you've scattered throughout the previous 200 or so pages. Endings need to be logical - think of the old westerns, when the wagons were circled and it looked as though the Indians would win the day, when suddenly the US Cavalry arrived over the hills in the nick of time. As a child I always used to wonder how the cavalry knew the wagon train needed their help - they didn’t have cell phones then, so just how did they know to show up? This loose end bothered me enormously - don’t let your readers be bothered by loose ends like this. Make sure you tidy everything up, so that each hint you dropped is shown to have a a satisfactory place within your story by the end. If you are enthusiastic and lay too many hints, clues and red herrings, or you have a sub plot that doesn’t actually develop because you decided not to follow through with it, then eliminate it from your story. Be ruthless. Stephen King uses the phrase 'Kill Your Darlings' for this! You want your reader to feel satisfied when reaching the end of your book, and think: Well, the ending was a bit of a surprise, but gee, I can see now it was really the only way this story could end!
