Stephenie Meyer had a dream, wrote it down, expanded on it, and sold it in 6 months.
I'm pretty sure J.K. Rowling once said that the entire plot of Harry Potter just fell into her head one day.
Does that ever happen to any of you? Me, neither.
That was the beginning. It took years to figure out what the plot would be.
My current WIP actually started with a scene that kept bugging me until I wrote it down... but the scene won't fit anywhere in the current book. The character in the scene won't fit into the book. I have all new characters. The only thing that's at all similar to the original scene is the magic system.
The next idea that's niggling at me deals with challenging established class systems. I have a vague idea who the hero and heroine might be (as in their societal roles) and I'm trying to flesh out the magic system... but I have no clue what they want or how the magic might help.
Nope, my stories come through blood, sweat, and tears. False starts. Deleted chapters. Rewritten endings and trashed beginnings.
How about you? Where do you get your ideas? (Might as well practice answering this question now, before you get published.) :)