Paperclip your return envelope to the story. Otherwise I advise sending the manuscript without a cover letter. But DON'T send your only copy of a story. Always keep a backup copy at home. Accidents happen with the post.
Editors occasionally spill coffee on manuscripts.
Start writing your next story. Whenever a story is rejected, send it out again to another market. Don't let the rejection stop you from working on your next story. I have heard of people who sold their first submission, but for most new writers rejections far outnumber acceptances. It does get easier as your credits build up, but the first few sales can be very elusive.