Besides taking classes over creative writing my top tips are: read, write, and accept critique. As I have written over the years I have watched each story get progressivley better. Much like visual art as you write your work on your techniques and really get to know yourself. Reading is the number one thing I have heard writers in interviews answer as a key to writing. Read things that you would normally read, read things you wouldn't normally read, read books over writing, read fiction, read news papers. Reading increases your vocabulary and you will find yourself picking up on writing techniques from the authors. Most importantly make sure you don't passively read. Most people passively read when they sit down, taking in the overall plot but not the finer aspects. You may have taken those classes in highschool where you have to write in the margins of books or in a comp book. If you are like me you had no idea why. Well, it is to get you more engaged in the writing and pick up from the author's message. Finnally listen to people, even people you are pretty sure are wrong. You don't have to use their advice, but by paying attention and considering their input will give you insight into yourself. You can still decide how much stock you put into people's opinions. Different views on things can only help you.
I don't have personal experience with autobiography apart from a few essays, but I would say a safe bet for me is breaking out the old comp book and pen and just writing. No editing, no stopping to look it over, just write. Helps me to start any project.
Oh, I thought you were writing a book. In that case, since it's so short, just keep if very focussed on key events and don't try to put too much in. And start with an interesting event instead of "I was born on blah blah".
I'm going to sound like a parrot here, but the best way to improve at your writing is to write more and read like a fiend. Know your genre. If you're writing about Edwardian England, read every book you can get your hands on pertaining to Edwardian England. Read novels and, also, make sure you do your research before you sit down and write. Of course you may have moments while writing where you realized you didn't look into something close enough and need to do a little bit of research, but that's normal. Also, read things that have nothing to do with what you're writing about. Read the classics. Find out why they're still in print and selling. Read quality literature and read trash. Get some life experience! That also helps.
And when you do finish something, give it to people to read. Not your parents, though. They're going to love what you write no matter what it is. Find some people who can give you constructive critiques (you'll know the difference between constructive and destructive). Have you considered joining a writer's group?
I'd suggest reading Anne Lamott's "Bird by Bird". It's quite inspirational.