I want to write a book but I don't know where to start. I'd like to know if is it there any structure for a book to be written, or if somebody could tell me how to start.
There are LOTS of ways to structure a book. Depending on what you're writing, there may or may not be any expected frameworks.
- Long-Form Fiction, such as speculative fiction novels, movie scripts, and semi-fanciful "alternate histories" usually progress lineally along one or more character's perspectives along a three-act progression, though more acts are non uncommon and strict adherence is not necessary. Some genre fiction have more expected structures.
- Non-fiction Reference Material, such as a wikipedia page or the manual for a piece of software, tends to begin with an overall review of the topic, follow with specific details or instructions arranged in some discernible order (complexity, alphabetically, or historical progression), and end with any needed reference materials.
- Non-fiction polemics and short-form fiction both are highly variable in their form and structure. Aside from the mechanical need to quickly attract a reader's interest, their choice of structure is entirely part of an author's craft and can be as important as word choice or phrasing.
As for how to actually write any of the above: start with an idea, add in various notes, and then sit down and write a first draft from beginning to end. Skip ahead if appropriate. Once your draft is done, read it once and then begin your editing process. Be aware you may need to throw it all away and start again with a new first draft, on possibly an entirely separate project.
Here's a terse summary of Peter Seibel's How to Write a Book blog entry:
Step 1 Write the index.
Step 2 Write a hierarchical outline.
Step 3 Write a flattened outline.
And Step 4 Write the book.
Seibel has written two technical books and you may take his approach as especially practical for a technical book.
It depends on the kind of book you're writing.
Fast-paced, action-packed books tend to leap straight into the thick of things and introduce their characters at the same time (Matthew Reilly kind of books. I know, he's not exactly an avant garde writer, but he is one end of the spectrum when it comes to explosions and many, many exclamation marks).
Modern romance novels that are highly successful tend to have the romantic interest pop up in the first chapter in a cute meet (forgive me if I have offended any sensibilities) with the protagonist or narrator.
If you're actually talking about what to put on the first page of the book, you could start with describing a particularly vivid mental image that sets the backdrop for the first scene in your story. Your imagination should take over from there.
The overall structure of books that I've read tends to go like this: Introduction (start slow, introduce characters, set tone for the story), buildup(things happen, information gradually becomes available), climax(the highest point of tension in the book or the main event all the other events have been building up to), and resolution (or the ending).
Don't let that formula confine you too much. Some of them have an ambiguous resolution (I know they call it a resolution, but the end of Twilight leaves you wondering if Edward turned Bella - more of a teaser, really), and some of them have the climax and resolution on the same page (the Notebook: The protagonist's love interest finally remembers him, which is the miraculous event you've been unconsciously waiting for all this while, and then the novel ends- Climax and resolution on the same page).
A shortcut would be to look up a few bestsellers that you think are similar enough to the book you want to write in terms of genre, mood, pacing and length, and then narrow it down to one or two of your favorite ones (or all of them, if you are like me). Use them (or it) as a guide, but don't overdo it, or all you'll get is another piece of fanfic. It would really help if you had strong original characters with quirks and idiosyncrasies that your references do not have.
Enjoy your journey!