I freely admit that this is fan fiction. I'm also considering a story or two based in a universe that already exists, just because again, having been reading since I was 4 years old, in the last 50 years I've read hundreds of thousands of stories and books and those universes are just out there.
Tell the stories your comfortable with, and which come from the heart. If you appreciate a particular sub-genre, that's a natural jumping off point. Just try to make it unique, distinctly yours in some way, so it's not just 'more of the same' and that you're telling a new story instead of rehashing other stories.
The best way to do that is to look at where others have fallen down: looking for stories which didn't quite work and trying to improve on what they overlooked. That's what inspired many of us at SOL. We read one too many stories and thought 'I could do a better job than that', and instead of just gripping we tried to do just that.
By focusing on a unique story, you begin looking at the story as a problem to be solved, rather than just a series of scenes linked together. You look for existing plot weaknesses, and figure out how to overcome those, while also developing the plot and building the characters. Those are all essential training for making stories succeed.
Having read a lot of fiction helps, non-fiction not as much, as the skills aren't as easily switched. As you've probably gathered from here, there's a lot of techniques used in fiction which won't work in non-fiction, and vice versa.
The biggest caveat, though, and this is for all of you looking to start writing and not just StarFleetCarl, is to think of the overall story, where it's going, how to develop it, pace it, keep readers captivated so they don't set it down and forget about it, and making the characters come alive.
As far as having the time to write, while it's an all-consuming passion, nothing demands you write a chapter a week for the next 52 weeks. Start slow. Invest the time before you ever put pen to paper to organize your plot and figure out how the story will unfold, and then write what you can. Chances are, you'll end up tossing several chapters right off the bat, but each revision will advance and improve the story. Each mistake reveals what doesn't work, and helps you understand what does.
The key, however, is not to procrastinate. If you delay, you begin to dread the work. The longer you wait, the more difficult it becomes. If you don't have time for a full chapter, try for a piece of one. If not part of the story, write a few interesting scenes you observe around you on any given day just to keep in practice--which you can always incorporate into another story.
Most importantly, if the story doesn't write itself, it's time to step back and consider where you went wrong. Often, writer's block is your characters telling you that your story has gone off track and you're asking them to do something they wouldn't likely do. Once you identify where you went off track, the words will flow once again.
Then again, if the words falter, waste a few hours writing forum posts which don't advance your story either, but at least you're practicing your craft.