If your writing is atrocious, or if you just need some help getting your thoughts in order, consider hiring a ghostwriter or a professional personal historian. Celebrities do it all the time. There is also a software that allows you to type your answers into a template on your computer, thus solving the problem of less-than-perfect handwriting. Many people also choose to type directly into an online template.
Create an outline or a graphic organizer of the major topics and ideas you want to cover. For example, you might make a time-order chart -- a chart with event blocks to organize events chronologically. Or, you might draw a cluster web -- a diagram with connected circles -- to divide your autobiography into sections, such as one for relevant background information, one for life-changing events, one for special memories and one for goals and dreams. Focus on reasons why specific events, situations and circumstances were important to you and how they made you feel.