The Speculative Element
I originally had planned to make today's exercise a "What If?" about your society, but I'm thinking at this point that there's only so much you can do with it before you write, and it occurs to me that there's no better way to start building your speculative element than by asking yourself "what if?"
What do I mean by "speculative element?" In a high fantasy novel, it's the magic, or the gods, or the One Ring. In science fiction, it's the hyperdrive, or the ansible, or the near-light-speed, or the wetware implant. In horror, it's the ghosts, the nightmares-are-real, the Blair Witch. Essentially, the speculative element is the rule or assumption that is not true in present day Earth.
And "speculative" pretty much means what if, doesn't it?
So, settle in with your notebook. If you don't know yet if you're writing science fiction, fantasy, or horror, you should probably decide now (and yes, a blend is a-ok; just be sure to do all the speculative element exercises once for each genre). But the chances are, you have some idea already of which genre(s) you want to work in. You might even have some specific ideas to play with-- a talking octopus, for instance, or a magically-appearing cat.
Write down the snippets and images that you already know you want in your story. Some of these might be things you have already decided on as convenience items-- a faster-than-light drive, maybe, or telepathy; things you want in there to help drive the plot forward and eliminate certain inconveniences as a storyteller. Just remember that these convenience items are always available; it's implausible to have the spacetime drive fail just when the hero needs it, and these things ought to be available to the villain, too, just so your heroes are playing on a level field.
Next, ask yourself the hard questions. "What if?" What if my society could talk to each other instantaneously? What would that do to them? Would that change my plot? What if you could make yourself invisible? Would that require magic, or technology, or both-- and how would you do it with either?
Last year, I knew I wanted aliens and a network implant (like having the Internet in your head); as I played with all my "what ifs," I realized that the network implant wouldn't just change the big things, like how dangerous a hacker could be-- it also changed the little things, like very few people carrying a briefcase or cell phone, and how you always would know where you were and could never get lost.
Drill down on the big things and the details, decide what would happen if they work one way vs. another. Chances are, you'll find there are speculative elements that you need in your story to support or limit the power of the ones you've already created.
By the way: I personally find that talking about my story with someone else helps me find the holes in my spec element. For the past week, my husband and I have been playing "what if" with my dinosaurs, figuring out what needs to change in their physiology to make them able to cooperate with each other as a society, and discovering that having twelve sentient species of dinosaurs means a very complicated society indeed.