When people ask me this question, what they're often really asking is "How do I get started?" Unfortunately, there's no guaranteed route. Every writer you ask will tell you a different story, usually involving some amazing and unrepeatable bit of luck or good timing. But, since I always find those stories interesting anyway, and since you asked...
I studied English and Psychology at Duke University, with the vague intention of pursuing journalism or law. The idea of turning my love of movies into a career never occured to me - until I met someone who had done just that. A real life producer, Thom Mount, was a guest speaker in my film theory class, and it was like being struck by lightning. Not because what he said was so profound (I don't actually remember any of it), but because here was an honest-to-God grown-up who got to tell stories for a living. A few weeks after graduation, I was driving across country to make it big as a kick-ass action movie director in Hollywood.
I ended up working for Kathryn Bigelow, the only female director making action movies at the time. Have you seen "Near Dark? Genius. I was Kathryn's development assistant. I read piles and piles of scripts and wrote coverage on them (a really useful skill for a would-be screenwriter - read one of the many books on the subject). It was a great education. I got read all the hot scripts, most of which were surprisingly bad - I was surprised, anyway. Doing coverage made me analyze why they were bad, or good, or unmarketable, etc. The job also made me realize two things. First, I didn't want to be a director. Second, I didn't want to be a development executive. I didn't want to work on other people's stories - I wanted to tell my own.
Right about this time, I discovered that the producers of "Star Trek: Next Generation" would accept unsolicited spec scripts (not submitted through an agent or lawyer). This was huge. No other show did that, and none do it now. I loved "TNG," so I decided to try. I wrote one and sent it in, and enjoyed the experience so much that I also wrote a spec episode of "Deep Space Nine," which was just starting up, and sent that in. I had found my format. I'd taken a stab at a couple of screenplays with disappointing results. TV just suited me better. Off those scripts, I was invited in to pitch "DS9." I eventually pitched there six more times, with four or five stories at a time.
Now here comes the good luck/timing. One of the "DS9" writers, Rene Echevarria, happens to be a Duke alumn as well. I ran into him at a Duke in Hollywood function, and he mentioned that the writing intern who was supposed to start on the show next week had bailed - would I happen to be available? Hell yes, I was. I had left my job with Kathryn a couple of months before to focus on writing. I don't believe in "things happening for a reason," but if I did, there it was. The writing internship is the best gig in Hollywood. You hang out in the writers room, attend production meetings and visit the set - and get paid. Again, not a lot of shows were hosting interns at the time, and I don't know if any are now. The experience was terrific, and culminated in the showrunner, Ira Behr, hiring me to write a freelance episode based on an idea I had pitched. I was in. My first screen credit. Membership in the Writers Guild. And I was writing for Star Trek!
My script, called "Hippocratic Oath," turned out well. The next Trek incarnation, "Voyager," was getting started and, as it happens, hiring writers. Ira passed my script along to the producers, Michael Piller and Jeri Taylor, who liked it well enough to offer me a staff job. I spent the next three seasons on "Voyager," writing fourteen episodes and moving up from staff writer to story editor to executive story editor (a silly-sounding title if I ever heard one).
So that's how I got started - through an amazing open-door policy that no longer exists and a lucky encounter with a fellow Dukie. I hope the encouraging part of the story is the fact that although I got a lucky break, I was ultimately hired because the Trek people liked my work. If you're good, someone will recognize it. The trick is finding that someone.