I suggest you start first by writing your story for yourself - like a journal. Write different bits as they come up for you - they dont' have to be in order, no one else but you will be reading it. Getting it out there can be enormously rewarding emotionally. Then, when you get to the point where you feel like you've got it all out, you can start going through your journal and picking out the bits you want to share with others. Then rewrite those parts bit by bit thinking about your audience and what they will want to know. Good luck. Your public library will have or should be able to get some books in for you about how to write your biography or memoirs.