How do I start writing

Many people are told they should write a book because of their life experiences. It's so easy to agree and be motivated to tell the world your story when you know that your struggles may help someone else...The bigger problem is where to begin. I have so much to tell, but organizing my thoughts is like trying to walk on water...Ok, maybe that is a bit of an extreme analogy, but that's how I feel about it. I have so many thoughts in my head that they get all jumbled up and it's hard to put them down in an organized fashion. My life is full of ups and downs, heartaches, and seemingly irreparable damage to my well-being. I just really want to get it all out of my head in hopes of healing myself and potentially helping someone else who feels they are alone in their struggles. No one has the same story; there are always differences based on multiple factors. Some stories may seem the same, but people handle emotions differently and come from different cultures; that makes a world of difference. I have a title, dedication, and even the beginning of a first paragraph for my book, but then I get stuck because I don't know what story to begin with first. My opening paragraph is more of an introduction for my life story, and so it isn't anything too revealing about how my life has gone to this point. The most challenging part of writing for me is how to begin; whether it is a research paper or a short story, yet I can begin a blog easily without much thought. Perhaps that is why when I last mentioned writing a book, I said how I wanted to write it like various journal entries. Blogging may come fairly easy for me, but I guess my issue with beginning my book is trying to protect personal matters without revealing too much about myself. It takes a lot to bare one's soul on paper. I have a blog that is password protected because I revealed so much about myself that I didn't want to share it with everyone on my facebook friends list. Does anyone know how I can motivate myself to begin a book that I am desperate to write? I need to get this stuff out of my head; I'm tired of it taking up space in my memory. I am trying to be more positive on my outlook, but it is most challenging with all the negative happenings in my past. I'm open to any suggestions. Once I begin it will be much easier. I've written 9 page research papers in a matter of hours; i just needed some kind of motivation to get it started...Writing about real life is much harder because there isn't any research involved in telling one's own story...