Every market has their own guidelines. Find a link called "Submissions" or "Submission Guidelines" or something along those lines. Then follow the instructions carefully. Please be sure that your story is of the right genre, wordcount, and format. Most of those rules are in place for a reason, and the ones that aren't are there to make sure people have read the guidelines before submitting.
In general, double-spacing, 12-point font, and a nice standard font like Times New Roman or Courier would be a good default format. Put your name, address, contact information, and wordcount at the top, since most places want that anyway. (I can only think of one off the top of my head - Flash Fiction Online - that explicitly doesn't want author name and info at the top of manuscripts.)
Go to Duotrope and/or Ralan. Take some time to figure out how they work and then use them. You might also want to keep a database yourself of what stories you've submitted to which markets when.
Write a polite and professional cover letter in which you provide your name, the title of the story, and the wordcount of the story. If you have any *notable* publications, then include those. (That means that no-pay or token payment markets, with only a few rare exceptions, are at best irrelevant as publication credits.) Please do NOT include advertising copy, nice reviews your mother wrote, or any other extraneous information. DO NOT ATTEMPT TO BE FUNNY. Copy and paste it when you submit, altering details where appropriate. That way you don't end up with awkward goofs when submitting.