First thing's first: master grammar and spelling. You can bend the rules of grammar stylistically, but making grammatical errors out of ignorance ruins your credibility. Best way to do this is to get a GRE or GMAT prep book and learn grammar from there. That's the foundation.
Second: read often, and read writers with distinctive voices and styles. It doesn't have to be boring, you can read fun writers like Pahlainuk, Bukowski, Hunter Thompson, Vonnegut, Brett Easton Ellis, or Dave Eggers. Sample different styles and subjects. Challenge yourself with harder and more esoteric books like Danielewski and Pynchon. Read books on philosophies you disagree with, like Rand or Camus. Like pickup, it can be daunting when you first start out, but if you get through that initial period of discomfort you'll find that you can't imagine life without it.
Third: when you come across a word you don't know, look it up. Try to use it in day to day life. Learn to find the fun in words (you know who's really good at this? Rappers. I love hip-hop). I've been digging the word "obsequious". Remember that human beings learn and structure knowledge by using webs of association, so try to apply your newfound knowledge of words and english to your day to day life and thoughts.
Fourth: write often and freely. Play with your own personal style. Like a baby learning to talk or a guy trying to learn guitar, you're going to start off by largely imitating those who came before you. You'll eventually begin to form your own voice.
