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.
The basics really are, as everybody has said, read more and write more. But there's more you can do to help yourself.
Get feedback. DeviantArt can be a good source of feedback if you're willing to get involved in groups and give others feedback, too.
Think critically. From now on, don't just read books or watch movies or sit in a theatre and let yourself be entertained. You won't get better if you switch your brain off. Analyze style, deconstruct narratives, consider what you would do differently if you were writing it.
Read the kind of books that you like to write. If you want to write an autobiography then read autobiographies. If you want to branch into horror then read horror novels. Also, read books about writing and literary theory.
Learn from other writers. There's a wealth of experience in different writing fields to be found just here on the dA forum. Many of us like to be helpful. Talk to people, listen to them, ask questions, join conversations. Some writers also keep blogs on the topic of writing and you can follow them.
But always remember that no two writers are the same and what works for others may not work for you. All the advice you get should be considered before adopted. It's all part of figuring out how YOU can successfully write better but I guarantee that if you're consciously trying, then it will happen.
There isn't exactly a shortcut to better writing, I'm afraid. You've just got to put in the hours and try your best.
For the autobiography, you might want to keep a diary, if you aren't doing that already. All the pro tips in the world won't help you if you can't remember what you're supposed to be writing about. I know that would be my problem if I tried to write one.
I like to carry a little notebook and write down anything that stands out to me. I write down all sorts of things my friends say. "It's always fun when inanimate objects start bleeding." "I just turned my red mage into Elvis!" "Do you want me to stop asking questions? Wait, that was another question, wasn't it?" I make lists in there too.
When in doubt Google. Seriously, the internet is your best friend for research. Dictionaries and libraries also are valuable resources use them.
Read. I seriously can't stress this one enough. Read anything you can.
Live. Personal experience is a good teacher and a great tool for any storyteller or autobiography. Get off the computer, out of the books and do something. Plan, prep, improvise whatever, just do something... within reason. Go to the beach, drive a dune buggy, learn to crochet, learn to cuss in Polynesian, pull some crazy prank on your friends.
*optional* Invest in good quality coffee. a good Guatamalan blend will jolt your senses out of boredom no matter how sleep deprived you are.
read more, practice more. if there's something specific you need help improving with, google that need for help. take responsibility for your own improvement. coming here for help for something that there are thousands of books and hundreds of thousands of articles that can give you what you need INSTANTLY looks lazy, believe it or not. rely on yourself more as you learn what to do, come to us when you need find-tuning on what you've produced.
How can you improve as a writer, by writing. That is the only way one learns how to write and improve as a writer. Also reading as well, read a lot. Then learning to look up stuff on your own is another. Autobiographies aren't something that I would say the average writer on DA would work on or would even study unless they themselves were interested writing one than say reading. You doing your research would probably give you a better answer than anyone on here could.