So in this post, I'm going to focus extremely broadly on the narrative structure of most books (and, incidentally, most plays, and even many short stories and jokes). In future posts I'll cycle back into specifics: fleshing out characters; symbolism and imagery; developing and resolving conflict, etc. But for now, let's stick to the basics: building a plot arc.
Plotting books is tough. It's definitely, for me, the most difficult part of the process. But it's a critical part--otherwise, it's all too easy to write yourself into a wilderness and become completely and irreversibly lost. (Not that, um, this has ever happened to me.) Understanding the basics of narrative before you begin will give you the equivalent of enormous sign posts poking up above the trees; no matter how deep in the forest you go, at least you will always be able to consult those signposts for orientation.
Okay, so, most books can be thought about in terms of a three-act structure. I find the 3-Act model incredibly useful when I am plotting books. It is simple and breaks down as follows:
Introduces major characters, the setting, and often backstory. Ends at the moment where the conflict the rest of the book will explore is established. (I usually think of this as an "inciting incident"--see my previous blog post about this here.) For example, in Romeo and Juliet, Act One would end after Romeo and Juliet have met, fallen in love, and discovered that they are the respective progeny of their families' sworn mortal enemies.
Act two comprises most of the developing action of the book; the additional conflicts that result from the inciting incident; the development of additional story-lines and complications; the attempts of the protagonists to bring the inciting conflict to resolution. It is important to know that Act II should end at a point of MAJOR dissolution; everything should explode, fall apart, seem irreparable and hopeless. In a traditional romantic comedy, for example, this would be the moment where the main character has (seemingly) ruined all of her chances for true love; the guy she is supposed to be with has moved away, or is on the verge of getting married to someone else; it will never work out, etc etc. In Bridget Jones' Diary, for example, the end of Act II is demarcated by Mark's departure to NYC and engagement to his legal partner--just after Bridget has finally accepted her feelings for him.
Act III is, obviously, the remainder of the book, and brings unexpected resolution to the seemingly insurmountable difficulties established in Act II--or, in a tragedy, does not so much resolve them as subvert them in some interesting way. (Plato and Aristotle would tell you this leads to something called "catharsis"--i.e. the you laughed/you cried part of the reading experience.) For some great examples of Act III resolutions, check out pretty much every romantic comedy that has ever been made or written. To stick with a previous example, in Bridget Jones' diary the culminating moment of Act III is embodied by that ridiculously awesome kiss in the snow. Hot, hot, hot. If you don't know what I'm talking about, please go Netflix that sh*# right now. Colin Firth 4ever!!
Okay, that was a little off-topic.
Point is: it's necessary to build a sturdy framework for your story. Otherwise, it's like building a really fancy house on top of faulty foundations. Add in all of the balconies, french windows, and flourishes you want, but sooner or later that pretty pile of plywood is gonna come tumbling down.