One of the tools I've used sometimes to help myself get started on writing is writing towards prompts or themes for upcoming publications, either anthologies or themed issues of periodicals. Since these markets tend to be open for only short periods of time, they're also good to keep track of for submissions of stories which you have already written which might fit more general themes. Below you will find information about three currently-open calls for submissions.
The one with the tightest timeline is Gaia's Misfits, an anthology of Young Adult Fantasy aimed at readers 11 and up. Submissions are due for this anthology by December 1st, 2012. They're looking for stories of 1,500-10,000 words and the payment is a flat rate of $15. There is some room left open in the guidelines for possible use of previously-published stories and the guidelines suggest that no exclusivity period would be required by the publisher.
Another submission call with an even broader theme is for Night Terrors III from Blood Bound Books. They are looking for previously-unpublished horror stories of 750-5,000 words and the pay rate is 5 cents per word. Submissions are open through January 1st, 2013.
Finally, Arc (a publication of the New Scientist) is currently running a contest for science fiction stories on the theme of "Is the Future Friendly?" The submission call is open through January 13, 2013. (The website says "before January 14, 2012." I presume the year to be a typo and since they say "before" I would be inclined to not push my submission to the last moment and get it in by the 13th.) They want stories between 3,000 and 5,000 words and will be paying £500 for one winning story and £200 for five runners-up.
I hope that one or more of these submission calls strikes you as being interesting. Best of luck with your writing and submitting!