I guess the short answer to this is that every idea I come up with starts with “what if.” Most of the time it’s “What if [insert something weird] happened?”
I think I come up with story ideas from all sorts of places. Anything can hit me at any time. I could be watching a TV show or looking at something online. Sometimes someone might tell me something about what happened to them and that sparks something in my imagination. I once got a vivid story from a picture my three year old niece made for me.
I write these ideas down so that I always have a stash of them in my back pocket just in case. I use Google Note, which is just a note taking app for Android phones and tablets. I really like it because I can write a note anywhere on any device I have as well as access my notes anywhere. So if I get an idea while I’m taking a walk or in the grocery store, I can just write it on its own virtual post-it for later.
I may come back in a couple of weeks or months when it’s time to write something. When I do there are always ideas that suddenly seem like duds and others that are still shining bright. And then I pick which one is calling me the most and write that one.