Now that we've covered some of the basics, let's talk about your creature's skeleton. It's what holds it together, after all, and on which everything else will be built.
The 3-Act structure is the foundation of most types of writing. It applies to fiction and nonfiction, short pieces and full-length books. It is, simply put, the Beginning, the Middle and the End.
Act One (the beginning) is where you set up the story. Hook your reader by starting out with a bit of action, or a gripping event that pulls him in and makes him want to -- need to -- keep reading. After you have your reader's attention, then go into the back story. This is a brief section that introduces the protagonist (hero/heroine), filling the reader in and laying the groundwork. Then you'll establish the central question around which the story revolves. Near the end of Act One you'll have an important Plot Point, a trigger that propels the story into the next stage, which is...
Act Two (the Middle), where the story truly develops. Your protagonist will go through a series of difficulties, each one greater than the last. He must struggle through these obstacles to reach his goal, advancing through both inner and outer conflicts. Outer conflict involves the plot -- it's what physically happens during the story. Inner conflict is your hero's personal demons or personality flaws that make him resist change. Act Two ends with a second Plot Point, which sends the story careening toward the climax.
Act Three (the End) builds steadily to the climax (the point of highest tension) of the story. Your character discovers his inner strength and confronts the antagonist, or the problem, becoming a better person for it as he heads for victory (or not). After the climax you'll have the denouement, a brief period in which the loose ends are tidied up or explained.
Act One usually takes up 25% of the story; Act Two takes up 50% of the story; and Act Three takes up 25% of the story.
Are the problems always tied up neatly at the end? Certainly not. Take a look at "Gone with the Wind" again. The Civil War is over and Scarlett has saved Tara. But at the heart of the story is the romance between Scarlett and Rhett. They're meant for each other, even though Scarlett spends most of the book pining for Ashley. Once Melanie has died you'd think the way would be clear for Scarlett to step in and stake her claim on Ashley, but at that point she realizes she doesn't want him, it's Rhett she's really loved all along. The book ends with Scarlett declaring her love, but with Rhett, by now fed up with her shenanigans, saying he's not interested and walking out of her life. In the final paragraph Scarlett tells herself that tomorrow she'll think of a way to get him back, because, "After all, tomorrow is another day."
These are generalizations, of course, but you should know about the 3-Act structure because it's an invaluable tool for putting together a book. Look for this structure in the books you read, and even in movies. Though not always easy to spot, it's always there.
"When I get a little money, I buy books. And if there is any left over, I buy food." Erasmus