Tina Jens has written an essay about cover letters at Black Gate. She lays it all out quite thoroughly. My two cents: If you have pro credits, you already know what to put in a cover letter. If you don’t have pro credits, Tina cuts to the chase.
Dear Black Gate editors, [If the market’s website lists the editor’s name, go with that.]
Enclosed is my story “The Orc’s Ascension.” It is 2,500 words long.
Thank you for your time.
It’s this simple.