Always enclose a self-addressed, stamped envelope for reply. Appearance and professionalism does count. Effective marketing is as important as good writing. Writing is a business, and only those who realize this will have a chance of succeeding. Use your query to explain to the editor why the readers will be interested in your topic and how they can benefit from your words.
Unpublished writers have the greatest difficulty selling their work.
You can make it easier to become published if you don't try for the national magazines right away. If you have to, write for the local shopper newspaper for free, but do the very best job you can do. No one will know that you were not paid for your writing and you will have published clips to show the editors of larger publications. Rejection is part of the process. No one likes to be told that their work is unacceptable, but it is especially difficult to have a creative endeavor rejected.