You cannot be a writer if you don't write so the only way to learn how to write is write something. You can start by keeping a journal or a blog or both. You might be wondering what's the difference between the two and why I am suggesting both. Living in a technology-driven world, these two terms might be synonymous. But just to be clear, blog is an online journal/diary and the journal I'm referring to is the old fashioned way - pen and paper.
If you want to keep a blog, it would be better to join a community like http://www.deviantart.com/. People there are very nice and they provide excellent feedback on your posts (poetry, short story, articles etc).
I've read from an article from a writers' magazine some tips to improve your writing skills. The author suggested to keep a journal (pen and paper type) and write anything you want to write. For example, you're walking and you saw a house. You could practice writing by jotting down descriptive words about the house then afterwards pretend to describe it to someone by writing it down. Pretend that you are writing to a friend and you are describing to him/her about the beautiful house that you saw earlier. Or you could try to write a make-believe advertisement, pretending that you are a ad copy writer.
There are several ways to practice your writing. Reading is also a good way to learn how to write. You can learn new vocabulary words and writing styles by just reading. You can improve your craft by reading blogs of well-known writers or by subscribing to writer-related materials. But still nothing beats practice. Write, write and write!
Sorry for a lengthy post. Hope my suggestions help you to become a better writer you wanted to be. Cheers! :D