So by this point, you're looking thorough the directories at http://www.duotrope.com/ in search of a market for your story. Each one of them will feature a set of submission guidelines (sometimes referred to as writer's guidelines). You want to read these very carefully. How well you follow these instructions will have a significant bearing on whether your work gets accepted.
Most of them list minimum and maximum word counts, so please make sure that your story falls within those parameters. Microsoft Word lets you check your word count easily enough (in the newest versions, its constantly visible in the lower left corner of your screen; older versions keep it under the "Tools" menu), so there's no excuse for not keeping track of it. Most publishers want Times New Roman, but some will ask for Arial or even Courier font; figure out what they want and make sure you use it. Do they want electronic submissions, or do they prefer "snail mail"? Do they want your work to be saved as a word document or in rich text format (.rtf)?
Also, pay attention to anything that the publisher says about content. If they come right out and say that they don't want profanity, explicit sexual material, etc., and you put that stuff in anyway, don't be shocked when (not if) you get rejected. I volunteer as a "slush editor" for Residential Aliens and Fear and Trembling. Though the guidelines are quite clear about not including vulgarity, you would be amazed how many potential authors try to send in stories peppered throughout with vulgarity. That failure to comply with guidelines results in a rejection every time (although we occasionally leave the door open for the author to submit a "clean" version, depending on whether we thought the story had merit otherwise).
Simply put, the guidelines are not suggestions. They are to be treated as hard and fast requirements to be followed to the letter. Do your homework and know what they want before you send your work in to them. Philippians 4:13.--SMS