If you want to write science fiction, a good place to start is by asking one of these three questions, and then see where it goes. Here are the questions, followed by examples.
1) What if?
This question is the most broad. This is the realm where anything can happen. Here are a few prompts:
What if space travel is commercialized?
What if we colonize the moon?
What if you discovered time travel?
2) If Only…
If you choose to go this route, there’s a good chance your story will end up in the alternate history sci-fi, or other-worlds sci-fi genre. Prompts from this question include:
If only the Soviets beat the United States to the moon.
If only a father could go to another dimension to bring back his daughter.
If only that plague could have been prevented.
3) If This Goes On…
Stories spawning from this question tend to have a social commentary bent. The author sees something going on, and speculates the inevitable conclusion. Prompts include:
If this goes on, society will live their lives in a fully virtual world.
If this goes on, there won’t be any trees left.
If this goes on, you could get arrested for walking outside.
My favorite book of all time, Fahrenheit 451 by Ray Bradbury, started when Bradbury asked the question, “What if firemen started fires instead of putting them out?” And the rest is history.
Jump. Build, Fly.
F.C. Shultz