John Smithers's comment is spot-on. Start writing now, and you can work on being a better writer as you go. It's like running competitively. Professional runners run a lot, every single day. They don't worry too much about if they're reading the right books about running, they just practice. Practice writing, then practice editing what you wrote, again and again.
That said, there are a lot of resources out there that can make a fine supplement to your writing practice. A couple of my favorites:
And finally, I would add that the second best way to improve your writing is to join a good critique group. This sounds scary to a lot of new writers, but it doesn't have to be. A good group is supportive and polite, while not being afraid to offer criticism.
Getting your work critiqued helps you improve, and it also helps you develop a thick skin. As a writer, you need to be able to accept criticism and use it to your advantage. And critiquing other people's work will probably improve your writing even more than being critiqued. It helps you develop a strong internal editor.
There are lots of groups out there. Some meet in person. If you're in a metropolitan area, there's probably one or two nearby. Otherwise, there are many on the web too. As a spec-fic author, I use Critters, one of the larger online critique groups. Conveniently, Critters is currently in the process of expanding to cover genres other than sci-fi/fantasy/horror, so you may want to check it out.