Step one is getting published. Technically, I guess you might argue step one is, say, learning the alphabet, but I’m assuming basic literacy and, ideally, solid spelling/grammar. Spellcheck aside, it’s worth learning the basics well enough to write a clean first draft manually. Therefore, step one is getting published, because it’s hard to judge your own work if you’re the only one reading it.
Note that by published, I mean, published by someone else. I don’t mean paid work, but I also don’t mean self-publishing on a blog. In other words, your work should be going through at least one editor who will hold you to a certain standard. Whatever your level of expertise, find a publication that you think you might be able to contribute to, and either query or just start sending in submissions, depending on their own publishing guidelines.
I started out with my university newspaper. Because writers graduate every year, typically you’ll see a general meeting which is open to everyone. Try to figure out which department or departments might be a good fit for the stuff you like to write, and start contributing. Even better, maybe your high school has a paper, but it’s likely to be published less frequently. Other options are school literary magazines, yearbook (more about photography and graphic design than writing, but still a possible in for something later), and newsletters.
If you’re not in school, start searching online. There are an amazing number of places looking for volunteer writers. Only a couple years into my school newspaper career I stumbled across an online review magazine, and sent in an audition review of a movie I had recently enjoyed. Two years of feature writing had honed my skills enough to get me on board, and they started sending me review material. Although this wasn’t a paying job, I was getting free product, which seemed pretty cool to me (school newspapers may receive freebies, too, actually).
Diversify if you can. Having a wide array of experiences makes it easier to customize your résumé to that job you really want. Any time a place you write for gives you a chance to do something you haven’t done before, breaking news coverage, writing for a different section, interviewing or profiling someone, that very piece may be the writing sample you pull up later to prove you can get a job requiring that skill.
But you can’t start building a portfolio of samples until you have someone to a) give you the assignment, and b) publish it. So get in somewhere, and write just for the fun of it.
Oh, and while you’re at your first writing job, there’s something else you should be doing: get better. Look at the writers, magazines, newpspapers, or whatever that you like, and strive to write something just as compelling. When inspiration comes, and you find you’re writing something a little over the top, just go with it, and edit afterwards.
Experiment with style, humour, and subject matter. Eventually you may write a piece and say to yourself, “Wow, that could have been published in [prestigious publicatilon]”, and it will be one of your go-to writing samples for later job applications.