“The things we have to learn before we can do them, we learn by doing them.” — Aristotle
Many scholars (much smarter than I am) have investigated how we learn and basically have deciphered these learning styles into three types: visual, auditory, and kinesthetic. (You can take a learning styles test here.)
I am a kinesthetic learner, which means I need hands-on experience. As a writer, the only hands-on experience you have available is to write. It is the act of writing that helps in learning the hows to and mechanics of good writing. But you have to start with something and have to get to that shitty first draft.
Summation: In order to learn, you have to do. To know how to write, you have to write.