There's an old adage that there's a book in everyone. True, but not everyone is a natural writer. Writing a story about yourself isn't as easy as telling it, but with a few simple rules and pointers, you can do it. It takes organizing your thoughts, getting an idea of where the story is going and, finally, just writing it.
Talk to a tape or digital recorder and tell your story there first. This allows you to start the process of thinking about the story as a story. It doesn't have to be perfect on the recording. The recording will give you a rough idea of what you want to say and the general flow of the narrative.
Listen to the recording and take notes. You will need an outline for the story before you write it. While some people do this in their heads, creating a written outline allows you to organize your thoughts, determine the story's flow and get started.
Start writing the story using your outline, but don't worry about the opening (the paragraph newspaper people call the lede). The lede is the hook into the story, the paragraph or two that gets the reader interested. Good writers leave the lede until the story is complete because at that point, it tends to write itself.
Simple is best. Don't use big words when they aren't necessary. In fact, don't use any words that aren't necessary. For example, "I said I would go" is better than "I said that I would go". Big words don't impress anyone, and they detract from the narrative flow.
Tell the story in a straight-forward way. Use description sparingly. "I walked into the tiny white kitchen aware something was wrong" is fine. "I walked sluggishly into the gleaming white but very small kitchen sensing something wrong on the wind" is way too much.
Be realistic. This is a story about yourself, not Clark Kent. Don't exaggerate. The real story will be far more interesting and believable if it is true without embellishments. Your story is interesting. Go with it.
Finish the story. Leave it for a month or so. Read it again and start revising. Rarely is a story perfect in the first draft. Don't be afraid to revise the story to make it cleaner, better, crisper. If possible, get a good editor. Friends and family aren't the best for this because they are biased. The best non-professional editor is someone who can write and can be impartial. Listen to this person's advice, but trust your instincts about your own story.