Here are the three places I start:
1. Dream -- occasionally I'll have a very vivid dream, or a very memorable if not quite as vivid dream, and make notes as to what happened, what was said, who was there, etc. Then I'll go back and fill in the blanks. Sometimes this means crafting an entire story to fit a single scene, as is what happened in a recent story I wrote. (Non-SF.)
2. Idea -- if I have something that I think is a good idea, I'll start writing about it. That happened with the horror story I recently finished and have sent to a writer friend to critique.
However, almost everything I write starts with the next one:
3. Dialogue -- I love to start a story by dropping the reader in cold. Starting with some sort of dialogue, I usually do 1-4 lines, and then start adding in dialogue tags and scene framing information. This is how I started my as-yet-unfinished (about 40% through the first draft) fantasy novel.