Ideas for a writer most often come from the world in which he or she resides. And, sometimes they jump out at you in bits and pieces, so try to write down every “bit” and every “piece.” Part of the puzzle of writing is fitting those bits and pieces together at a later date. I think it is a mistake, by the way, to go right to work on an idea – you should let it percolate in your brain for a while. It will either get richer and grow in stature, or you’ll realize that it’s an idea that needs more growth and depth. (In other words, “Wow, does that stink! What was I thinking?”).
But, when those ideas come to you, write them down. As the author E.M. Forster once noted: “How do I know what I think until I see what I say?”