A lot of sci-fi/fantasy (okay almost all) books have their own world created for the book. Even if this is not how you typically do things, I want you to write a few paragraphs describing you r world or a list of things (i.e. types of people, language*, plant life, sky color, land masses, bodies of water, etc.). This does not mean, necessarily, that you have to make up and alien planet with skies that look like tie-dye at sunset and no one speaks anything but Pig Latin. I particularly am working on two stories that are set in made up towns that seem to be in America. So you can do this.
I personally like to make up where the characters live to avoid in-authenticity with any real place. I have not travelled a lot and would rather remain authentic to my own imagination. Plus it is fun to create a whole new place and have it be 'just so'.
So. Get out your writing notebooks, open a new word document, or log in to your online journal and get writing. This doesn't have to be in story form (although it would be a good opportunity for you to try working on setting the scene in a story); you can write a list, draw a map, etc. I just want you to try to work your imagination and try to create a believable world that never existed before you dreamed it up. Try not to do anything cliche like an underwater city hidden for thousands of years wherein the inhabitants speak perfect English and are a thriving community. okay? Atlantis is out!
As always we'd love to hear what you have to write if you want to share some or all of what you create from the writing prompt. Your works is safe from cruel unnecessary critique and any critique at all if you so choose. Leave a note if you would like feedback (but remember any feedback given is subjective. Personal taste accounts for so much when one is critiquing another's work).
And last but not least, happy writing!
*Quick word of advice: I was at a writing conference where I heard a sci-fi author say to not make up words lacking syllables. Apparently this is frowned upon and I would not want to be held accountable for encouraging it!