What is science fiction?
There are lots of definitions of science fiction out there. But basically, science fiction is the end of the equation: reality + what-if = ...?
In science fiction, the way this what-if element operates is based on the principles of actual science. For instance, if there is time travel in a science fiction book, it is performed with technology, not by waving a magic wand.
Science fiction is for you if...
Creating imaginary worlds
Science fiction writers create imaginary worlds. These might literally be new worlds, i.e., an invented planet. Or these might be our world in the future, or with some innovations (for example, that humans have evolved to breathe underwater).
The way things work in your imaginary worlds will be based on actual science. So it's important for you to be familiar with the scientific principles and inventions that are related to your creation. For example, if you're writing about humans living on a planet with zero gravity, then you need to know the effects of zero gravity on the human body and the kind of technology that would be needed to compensate. If you write a novel that takes an existing scientific discovery one step further, then you have to understand the actual discovery.
Then you have to figure out the exact rules of your imaginary worlds. And you have to follow them.
Your prompt: 1) Start with a "what if" statement 2) Choose your world 3) Enter that world and write.
adapted from <http://www.creative-writing-now.com/how-to-write-science-fiction.html>