I started (not counted school assignments) when I was in fifth grade, mostly cheap Star Trek ripoffs, heavy on "Marty Stu", oh, one retelling of Tom Sawyer that was practically an abridgement. I didn't finish any of them. Really, I was trying to write novels and just didn't have grounding for that.
My mother suggested that I try shorts but for some reason I never went anywhere with those back then. Then, in the summer of 1977 (between my Freshman and Sophmore years of high school) I finished my first piece that was relatively substantial. A screenplay. A science fiction screenplay. Okay, it was a ripoff of Star Wars. It was a bad ripoff of star wars. Written entirely by hand (I didn't have a good typewriter at the time) there was only one copy in existence which was soon lost. I wish I knew where it was. Because, you know, if I knew where it was I could destroy it. So long as I don't know, the specter of somebody finding it and threatening to release it to the world unless I perform some unspeakable act for the finder hangs over me.
It was bad.
I was back to partially done stories for a while but at this point I started looking seriously at shorts. I read collections (had not discovered the magazines yet) from the library--the "Orbit" anthology series, the "Nebula Winners" and others.
Then, in my senior year, I started writing a new piece. It grew and grew. Five hundred pages (still handwritten, but I had a rather small hand back then so it was novel length) I had a completed manuscript. It was still bad, but it had some ideas in it that I may revisit someday.
From then I went into the Air Force. I started writing more while in, not so much while I was in training or assigned overseas, but when I returned to the US for my last two years I got serious about it. I started writing shorts. I started submitting them (I'd discovered magazines by this point). I started having them rejected.
It took another five years before I had my first sale. I sold a handful to Analog, one to the late Marion Zimmer Bradley's Fantasy Magazine, and a few non-fiction pieces. While not much, this was enough, in fact, to get me exempted from the English language requirement at the college I attended (only person ever to do so). But that's the thing, when I started college I really didn't have the time or energy (especially the energy) to write fiction much. Then after college it was job and work. For a while I worked on a webcomic (and I really suck as an artist). But it was only in the last few years that I got serious about writing again. I can't say I write every day, but I write most days. I have had a few professional sales (some we don't talk about any more ;) ) and a few pieces I've taken "Indie" which at least some people have enjoyed.
But that's what writers do. Writers write. Selling is a secondary consideration, a nice one, but not the core. To be a writer you must write.
So, when cons have that panel on "Mistakes beginning writers make" I almost always volunteer for it because I am one. I've just been one for the last 37 (or more) years.
Eventually, I learned that some of the worlds I’d created just dripped story ideas. There were just so many things I could do moving forward or backward in time or to different locations in the same world. And then I found, thanks to that writing book Sarah recommended, that I could sit down cold, pick some starting point (say, “I want to set this story on the Moon, in my FTI world during the colonization phase, and maybe have a teenage protagonist”) and just noodle around until I’d generated a “story idea.”
Finally, I’d reached the point where I no longer had to worry that I’d run out of story ideas, that the time would come that I’d have to say “I’m done” because I had nothing left to write.
Only took me 37 years. ;)