Writing a book can be a wonderfully difficult experience.
I have a few tips to help you get started:
Start small. You don't have to sit down and write a full chapter in the one day that you decide to write a book. Maybe just a few words to get you started, or you can write down all the ideas you have for the book. That's where my second tip comes in.
Have an outline. Have a relative idea of what you want to write about? Good. You should probably write it down. Not just the general idea though. You should write down something more in-depth, like the plot of the book. As you continue to write your book, you should write down every little idea that pops into your head, even if it sounds stupid. You don't have to use every idea, but it is good to have ideas written down somewhere in case you forget, or actually want to use it. You could always change the idea, morph it to fit your book. All of my plot notes start with "Have (such and such character) do (something)." They're stupid notes, but when I need to write fast, that's how I jot them down.
If you don't have the full plot figured out. It's okay if you don't have the full plot figured out! I almost never have my books totally planned (I'm just one of those writers). I can have the middle and the ending, or the beginning and the middle, or the beginning and the end of a book. As long as you know what you want to write about and have some general idea of an outline. You don't have to have everything figured out. The more you write, the more ideas you will come up with, so don't get frustrated.
Writing regularly. I highly suggest writing on a regular basis. Maybe you don't write everyday, but you should write a couple of times a week, if you don't have severe writer's block (you should try to write to overcome this). If you don't write regularly, you might forget where you are and what you were doing when you stopped last. It also keeps your momentum up. You are more likely to write in the same manor if you write more.
Deadlines. Giving yourself a deadline for each day, week, or month will keep you motivated. Lindsay Cummings tries to write 1,000 words a day. Stephen King tries to write 2,000. Everyone is different and only you know what you are capable of in a day. Sometimes it takes authors years to finish writing a book.
Read, read, read! You should read more than you write! Writing is important, but reading can do wonders for your book, and yourself as a writer. Reading can give you ideas (don't plagiarize though, obviously), can inspire you to write, and trigger your creative juices. When I'm reading a good book, I usually have to put it down so I can start writing. When I feel inspired, it feels like I'm overwhelmed with words.
How I think of a book as I write.
I see a book as a puzzle. There are all these little, intricate pieces that require careful observation before you try putting them in the slot you think they belong in. Example: Maybe you add some tiny detail half-way through your book that turns out to be very important at, or towards the end.
I think of chapters as a staircase. You move up a step after each chapter you complete, because in each chapter you are building the story, and building suspense.
As a writer, it is your job to make all of the pieces fit, unless you're writing one of those books that have a split ending where you want the reader to decide the ending, like in Inception. (Yes, I am aware that it's a movie but I couldn't think of a book).
And it is your job to build the story as you climb the staircase of chapters.
Don't give up. You will feel very accomplished when you finish your first book.
Write another. Don't get frustrated if the book you have just completed doesn't go anywhere. Maybe you try to get it published, or represented but you can't. That's okay! Many authors get rejected. J.K. Rowling's Harry Potter books were rejected by eight publishers. For more information on negative feedback, click here.
No matter what happens, nothing should discourage you if writing is something you love and want to do.
Editing your book. Click on the link if you're at the stage of editing: http://www.ashleyearleybooks.blogspot.com/2015/07/editing-your-book.html
Happy writing! Good luck on your book!