I'd say that the first thing an aspiring writer MUST do is get a good, REALLY GOOD handle on language. Learn the rules (then you can break them effectively!) Learn the conventions, styles, spelling, vocabulary, punctuation, grammar of the language, inside out.
I got an email from an aspiring writer asking "Would you mined looking at my writing sample? They're are only ten pages. Its a thriller, set in the future..." The three mistakes in the previous sentence are obviously NOT typos. This is a case of not knowing (or showing that you know) the difference between:
it's and its
there and they're
mind and mined
As boring as it might seem, you should get a stack of grammar books or take a refresher course.
An author once visited my book club and spoke to us about re-learning English. When he first set out to tell a story, he had no idea what a dangling participle was and couldn't tell a preposition from a proposition. But he recognized that if this writing thing was ever going to pay off, he needed to brush-up on language.
A friend from the book club later slagged the writer in a conversation with me. I didn't say a word to her at the time, but I mentally corrected her grammar about a dozen times during our chat (she never uses the past participle with modal verbs. If you don't understand what I just said, Google it).
I guess the point is that, even though you may read and speak and write in English all day, every day, you may very well be making mistakes that you're unaware of.
Second, you should know your genre. If you want to write science fiction, check out the GREATS (Asimov and Bradbury, I guess?) Read blogs about the genre and pay attention to fans' comments about what they liked (or not) about the latest hot new title. Learn who's publishing what kinds of titles. This way you can determine if there's a fit for you with that publisher's line-up. Learn the genre so that you can spot the gaps, and then offer something to fill them.