Not everyone who wants to write will make it; desire is no guarantee of talent, unfortunately. Writing for pleasure, for satisfaction, is the first step. Write because it’s as natural and as urgent as breathing. Write everything that occurs to you—fiction, nonfiction, poetry, plays. Try out everything and see where your instincts lead you.
But no matter what you write, the key is to revise it, to look at your writing and make sure there’s no spare word in it, no wasted descriptions, no fillers or bloated adjectives. Read it out loud. Does anything feel precious, inflated, monotonous or unclear? Deal with it fairly. If it doesn’t do its job, get rid of it. If it earns its pay, let it stay. If, as you read through your work, you find a desire to skip a paragraph or two, pay attention: that paragraph should be cut. You should never submit anything without having gone through a dozen revisions. Does that sound like too much? Does that mean your article won’t hold up when looked at closely?
And you should never send out anything with any kind of error in spelling or grammar. Never. Look at the Manual of Style at the library if you don’t own one, and just start skimming through the pages—and pay close attention to those areas where you know you’ve made mistakes in the past. Poor grammar and spelling put you in the kiddie pool of writing; you don’t want to be there. And unless you were an English major (and sometimes even then), you’ll probably benefit from looking at a style guide at odd moments. Clear writing improves any submission.
Get someone to give you feedback; if possible, join a writer's group. It’s not easy to find people who are honest and articulate, but that’s what you need. You don’t want yes-men, and you certainly don’t want those hypercritical types who see flaws and nothing but flaws. You want people who react intelligently to what they read, and who read, or at least are open to, the kind of thing you write.
And speaking of reading the kind of thing you write, that’s what you should be doing, too. A good writer is a dedicated reader. You should be well-educated in your field, whatever it is, and you should be current. Tastes change in the zeitgeist; writing like Arthur Conan Doyle because that’s who you like and you’ve never read anything written after 1930 is not a good strategy. Fate can be whimsical and maybe you’ll succeed, but more than likely you’ll just find yourself uncomfortably dated.
Read and be alert while you read—you can learn the most from understanding what, in a story you admire, moves you. What scenes work the most forcefully; what omissions actually tell you something? Pay attention to how your favorite authors use dialogue, how they use pacing. Re-read the good stuff and think about it; the more you see it the more you’ll see why it works. And that translates into being able to see what works best in your own writing.