A standard short story cover letter (not really a query) is a simple thing whose sole purpose is to mention any writing credits you have or anything else that might potentially be of interest. With a novel query, you are trying to hook interest to get somebody to commit to reading the thing; with a short story cover letter, you are just saying howdy because they will read the story no matter what (although some may stop earlier than others once they know it isn't right for their magazine)(and this assumes the story follows submission guidelines for the magazine).
Some magazines like potential bios to be included also, but that depends on the magazine. So read the submission guidelines very carefully and make sure you ave everything necessary! If you have no relevant publication credits, just skip that line in the cover letter. A lot of cover letters are just skimmed anyway. Some folk don't even read the cover letter until after they've read the story.
Example (a bit dry and sad, but functional):
Dear <Awesome Editor's Name>,
I am submitting "Great Story You'll Love", a <word count>-word <genre> short, for your consideration. My fiction can be found in <list of cool magazines you have been published in>. Thank you for your time.
Best wishes,
<Your name>
-Michelle