In order to get writing experience you have to write. First, write to yourself. Start with a topic you feel familiar and comfortable with, and write something. Say, a book or a movie review. Write some more of those. Next, you can open a blog. Say on Blogger, which is free. Set a schedule and write. Next, you can try to print some of your stuff - by sending texts to writing contests. A college course involving intensive writing may help.
Reading will also help your writing. Read about subjects you care about. Read for content and style. Another possible exercise is taking some piece of writing and rewriting it to your liking. Also, you can join a writing workshop.
I added links to some prominent bloggers' advices about writing.
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You become a writer the same way everybody else becomes a writer -- plant your butt in a chair and start writing. Writers are people who write.
By practice -- the more you write, and the more you pay attention to how you are writing, the better you will get.