8. There Are No Golden Rules
The worst thing a book can do is bore you. It's okay if it terrifies you, or even makes you depressed (some people like that), but if it bores you, you'll chuck it away. Unless your teacher is making you read it. So whatever you do, when you write a story, make sure it isn't boring! Easy to say, but how do you make a story non-boring? Here's the essence of it: create a main character the reader will care about, and give them a problem or mystery to solve.
If the characters are dull or annoying. without anything to like about them, the reader will get very fed up. Even your bad guys should have something interesting about them (Voldemort was at Hogwarts, for example)
If the character has a nice, easy life. everything is hunky dory, no problem, the reader will fall asleep. To spice things up for the reader there should be some some sort of conflict. a difficulty with someone (a bully, evil wizard, monster, annoying brother, parents) or something (the sea, a desert, loneliness, an approaching asteroid, time running out).
If there is no question the reader wants to know the answer to. he or she will die of boredom. You must keep your reader guessing - wanting to know what will happen next, what the answer to the big puzzle is. Whodunnit? Will the hero save the city? Find the treasure? Kill the monster? Win the girl's heart?
This is one reason why JK Rowling is richer than the Queen. She is brilliant at making sure the reader is always trying to work out what on earth is going on. You can't put her books down because there's always some mystery to be answered.
Clues and Red Herrings
But don't keep your readers totally in the dark - give them a few clues so they can make an intelligent guess. It's a delicate balance - the events and the characters in your story shouldn't be predictable, but what happens should make sense. A teacher may turn out to be a vampire, and this should be enough of a surprise to excite the reader, but there should have been the odd sign - a pupil with two little red marks on his neck, the teacher refusing to eat garlic in the school canteen, and so on.
You can also give red herrings. These are false clues, meant to lead the reader to a false conclusion, so it's not too easy to guess what's going on. Another teacher might be really mean and have big teeth, so the reader suspects it's he who is the vampire - but it's really the teacher who seems very nice and friendly (but doesn't like crosses. )
Your readers will have much more fun when they have a chance to guess what's going on - when it's not too easy, but not too hard, to tell what will happen.