Firstly I would like to appropriate some Douglas Adams and tell you
Don't Panic
Think of the spec fiction conventions: microcosm, right the wrongs in a world, parallels to our society and such.
The way I make up ideas for speculative plots is just by asking random 'what if' questions. something like:
"what if there was no colour red?"
"What if on every secon tuesday you weren't allowed to go outside for fear of catching the sphinkter virus (which can only strike on every second tuesday)?"
"What if the world were controlled by one giant man with a fur cape and 7 fingers on his left hand?"
Once I get a concept I like then you have to work out why it is like this, did some government take over, was there something apocalyptic, did aliens land and re-encarnate the dead?
Then I work out when your story is set. Is it during the event, in the time leading up to it (perhaps someone is trying to prevent it?), after?
Then work on some characters to live in our random-mixed up world. (ther conventions are useful in doing this - think hero, adviser, sidekick, absolute evil etc.)
Once you get some characters the rest of the plot often writes itself.
Be careful though, with speculative stores it is easy to think of large plots which need many thousands of words to do justice. If you are writing for an exam or an asesment you will ony have around 2000 words.
Generally a simple plot in a complex world works well. The good thing about spec fiction is you can leave it hanging. Just say that your story is the first in a trilogy or something (it's one of the conventions so you can)
I hope I was able to give you some ideas somewhere here....
good luck,
~ArtOfLosing