So, when I was eighteen, I set out to write a novel. I had no qualifications aside from a good command of English grammar. I'd read a lot of science fiction novels, but as it turns out, writing a novel is a lot harder than reading one. I picked up a lot of what went into those books, but that didn't include structure, plot, characterization, sentence structure, or any useful storytelling techniques beyond coming up with a really freakin' awesome premise involving an entire civilization and/or a woman in a sexy spacesuit.
I promptly fell into world-building purgatory and stayed there for over ten years. In 2009, I finally finished that book, kicked it out the door, and started on novel number two.
And, as it turns out, I hadn't learned a whole lot in those intervening ten years, despite taking two creative writing classes. I guess one has to actually write to learn how to write. In the last two years, I've written nearly 500,000 words. (That's halfway done with my practice words!)
I remember clearly the day I sat down with Novel 2 (In Memory) and proceeded to begin from as wrong a starting point as I possibly could. It's a day I'll never forget, and it's put me through rewrites and hard lessons I don't ever want to forget.
First, I had a good premise. What if there really were some kind of magic in the world? It would be clearly visible, yet completely unnoticed or disregarded. It wouldn't involve hidden worlds, fantastic creatures, or magical talismans. Etc. etc. etc.
What I lacked was an actual plot. Common newbie mistake.
Note to Self: Pick a single character who wants something, put something else in their way, and then give them a hard choice to make about how they're going to get what they want.
DO NOT: Pick ten or twelve interesting characters, give them each a cool new power, and dump them in a box to see what shakes out.
Only after you've got that one, central character can you even begin to think about subplots. Subplots aren't always necessary and are frequently undesirable, but they can be good if they: tie in to the main plot in a compelling way, have their own actual plot (see above), add to the story/theme/tone of the book.
I'm not an expert on subplots. For Novel 3 (Drivers), my first instinct was to have a main plot and a subplot, but I've already discarded that idea. There's something attractive about keeping things simple—especially after what I've gone through trying to get In Memory to a coherent form.