I'm new to writing and I'm still young (preteen young). I want some advice on writing and help.
How am I gonna start my book? What is the best way to begin writing a book?
I would suggest a different approach than the other answers.
If you are completely new to writing, just write.
Think about other areas of learning. If you just had your first physics class, the next step won't be building a car. The first thing will probably be something like a battery, two wires, and a lightbulb.
Writing is the same. Like anything it will need time and many failed attempts to learn it. So don't start by studying the handbook for automobile engineers, but try wiring an on/off switch.
Instead of a theoretical approach, I suggest to you a practical approach. Do not study how-to-write books. That theoretical overhead will only cause writer's block. It's like reading about gravity when you want to learn to ride a bike. Not helpful. Many aspiring writers have found themselves bogged down in character creation or three-act-structure and unable to write a single word. Instead, just write a book, or try to. From the gut. And if afterwards or while you are working on it you have a question, find the answer to that and implement it.
Those questions will be very specific to your current problem and level of ability, and the answers will be helpful and provide intense learning.
How-to-write books are a great resource of answers, but only if you apply them to what you already do. An answer to a question you don't have is confusing and distracting and will cause a problem instead of solving one. Also, people are different, and not everything that a writing coach says will work for you. That's why, if you have not yet had any writing experience of your own, it is better to not read how-tos and only find answers to what actually ails you. Later, when you have written a few books and know your writing self, get those manuals and study them. But not yet.
And when you are done with that first book, or have given up on it, get feedback, and then write the next book. Because it takes many unpublished failures to write a book that gets published.
If you don't believe in the principle of training – because that is what all those unpublished "failures" are – you don't have the professional attitude that leads to success. Wanting your book to be turned into a movie is a great goal, but it never happens to most published writers, and expecting it for the very first thing that you write is just unrealistic and naive.
You are not unique but one among a million kids with the same dream. The only things that can raise you above them in the long run are diligence and perseverance.
I don't believe in the advice to write short stories first.
Personally, I don't read short stories. I don't like that form. And even if I did, short stories are not short novels and don't prepare you for writing long fiction. They have a different structure, different topics, use different narrative devices.
Writing short fiction will certainly train your language proficiency. But you can train that by directly writing a novel as well. Writing short fiction will give you your first finished work faster, but writing good short fiction is just as difficult as writing good long fiction.
If you like short stories, by all means, write them. Write poems, write blog posts, even draw and make music and do sports. It will all make you a better writer, because writing is creativity and life experience and many things more. So exercising your whole being will make you a better writer. But don't believe that you have to do something before you are ready to write your book, be it reading how-to books or writing other literary forms. Because that is not true.
Whatever you want to do, do what you want to do, and learn it by doing it, and not by reading about it or by doing something else, that is the advice I give.
But there is one kernel of truth (for me) in the advice to write short stories that I want to point out because it is important:
Finish your first novel in a reasonable time.
Many aspiring authors get lost in endless cycles of revision, expansion, deletion, replotting and so on. I know quite a few people who have been writing "their novel" for the past ten years or more. Do not take that road, it is the road to becoming a failed artist and losing your self-respect. However you feel about your writing, do not attempt to make it perfect. Finish it, and move on to the next work. You can pick it up again in a few years and rewrite it, but the most important thing, no matter what you write or whatever art you create, is to finish one work, take a break (if you need it) and then begin the next.
That is the difference between a writer and a wannabe writer: the ability to move on to the next work.
You can take part in NaNoWriMo and write a novel in a month, or give yourself three years. The exact period does not matter (although half a year to a year are probably the most productive length and more than enough for a first attempt), but the important part is that you finish (or give up) when you feel that that was it. And you will note it. There will be a moment when you feel that there is nothing more to be written. It may happen when your novel is complete, or it may happen while it is still incomplete, but it will come, and it is important that you heed that feeling and let go and move on.
I agree with what: Actual writing and making your own mistakes is essential when attempting to become a novelist.
However, if you are interested in textbooks: Here's the first textbooks I've read when I was about 15 years old.
I haven't read it, but maybe the "... for dummies" series would be interesting for you as well.
Lastly, one thing that I can't stress enough: Figure out what you like about stories. Then write exactly this kind of story. How to do this? Exercise your writing. And read. A lot. No textbook is going to be of any use for you if you haven't read any (in your personal opinion: good) books that you can dissect with the means of the textbooks.
For advice on the more creative aspect of writing, see Tommy's answer. As for how to start from a structural standpoint:
The plot diagram of most stories is well defined, and a version of it can be viewed here: Plot Diagram. As you can see, it begins with the Exposition.
During the exposition, three things are introduced: Setting, Characters, and Conflict. Obviously, the length of the exposition and amount of detail will be defined by the length of the story as a whole.
A common type of story is the Hero's Journey, which also has a diagram: Hero's Journey. This follows the Initiate, who starts his journey in the Known World. In this case, the exposition mainly consists of introducing the Initiate, his or her "known world" (village/town, friends/acquaintances, goals, interests, etc) and then the event which pulls them into the unknown world (which is the conflict/inciting incident from the plot diagram).
As for style and creativity, that all has to come from you. Structure will only take you so far.
I hope this helped, and keep writing!
The best way to begin a book will differ from person to person. Some people, once they have a vague idea of what happens, jump right in and start writing. Some have to spend months developing character, stakes, and plot. Some (like me) are somewhere in between.
Because of this, the only way to answer your question is through practice. Practice writing short stories, and see if they turn out better when you develop first, or when you just write first. I have found that writing fan fiction is perfect for this. Find a site that supports fan fiction, or short fiction contests, in something you are interested in, and start writing (or developing, as the case may be).
Regardless of which way you write, the first thing you write will be the rough draft. The rough draft, for those that write first, is where you get your ideas down and see how they unfold. It's where you see the connections and details you had overlooked. Once you feel ready, work in some development, and start writing again. You may have to go through several drafts before writing the final one. Don't worry, this is normal.
If you develop more first, the rough draft is the place where you see how things unfold, and why or why not certain scenes and/or ideas work. I know from experience, that something can seem perfectly logical in development, and completely ludicrous in writing.
I've found that the more development you do, the fewer drafts you will have to write. This does not mean you should do more development right away though, since I've also found that my writing suffers when I do that.
Practice, and find out which method yields your best fiction. That is how you will begin writing.
I agree with much of what is said above and would add that you have to read. You have to read a lot. You will not be able to write unless you read huge amounts, both in and our of your chosen genre, and you read about writing. Another important point is that you must make effort in all your writing, everywhere, if you are to make it your craft, meaning grammar, punctuation, and style must become your lifelong passion and your best friends ... and just an observation that, from the way you asked you question, it seems they haven't yet entered the circles of your nearest and dearest. Being honest is another trademark; you have to write what you know for you writing to be authentic and feel personal.