You discover your own truth through experience, and this is as true of writing as of anything in life. Measure everything I suggest against what you find works or does not work for you, and allow yourself to disagree. The whole point is to start writing and keep on writing. Anything that puts you off needs to be ejected from your psyche, at least for a time, because it's the last thing you need and there are as many opinions about writing as there are writers. We don't all do things the same way, in the same order and with the same mindset, so what is a must for one can be a turn off for another. We're living in a fabulous age in which all kinds of people are finding all kinds of ways of doing what they want to do. If you want to write a book and publish it, you can, in your own idiosyncratic way; or by following the tenets laid down by experienced writers if that's where you prefer to start. Either way, you have to find and nurture your confidence if you are ever to allow your own unique voice (and everyone has one) to emerge and your words to have a life in the world. No one else lives your life or thinks your thoughts. Perspective is unique and no matter how small the remove, difference in interpretation can be incalculable. Only you can express your individuality, which is the base line of what you bring to the world that no one else can. The world changed the moment you were born: that's an irrefutable fact. 

Countless people want to write a book. Books have power, kudos, mystique and there are many reasons for writing one: you want to make money; you want to raise your professional profile; you believe you have something to say that no one else has or can; you have one or more stories inside you scrambling to get out; you think it would be a cool thing to do... 

Relatively few people actually do it because an inevitable barrage of resistance, in the form of doubts and negative thoughts, gets between the writer and the page: you don't have the time, energy or expertise to write a book; you don't have the arrogance to assume you can, etc., etc ...
Shakespeare said it best:
 
      Our doubts are traitors,
      And make us lose the good we oft might win
      By fearing to attempt.

A book is the same as a marathon, which begins with a single step and is run one step at a time; a book begins with a single sentence and is written one sentence at a time. When it all seems too daunting, make it manageable: bring your mind down to the first or the immediate next sentence. The greatest most prolific writer you can think of wrote one sentence at a time. It's all anyone can do.

Writing is no easy bedfellow. Its gift as well as its curse is that it turns us inside out. We have to love it to stick with it, and getting started is the hardest part. Recognise procrastination and avoidance in its various forms: checking/answering email, finding the perfect pen, notebook, time or place, or anything else that seems more important than getting down to actually writing. If you really have no idea where or how to actually start your book - or any piece of writing - and the how-to books aren't solving the problem, write whatever comes to mind, be it a synopsis, an event in the story, description of one of the characters, or why getting started is so difficult. It doesn't matter as long as you start writing, keep on writing and don't go back and judge/criticise what you've written. You're getting the flow going and the rest will come. Structure, organisation, editing can all come later.

Your book will only be written if you write it!