As well, always include a Self-Addressed Stamped Envelope. In the second paragraph, let the publisher know if you're already published. Some publishers want to print work from writers who can bring along an audience to their publication. It helps circulation. If you don't have any publishing credits yet, don't make them up.
Editors have access to reference resources that list what was published by writers in the press.
There are other writers out there making their living by creating those resources. Remember to put in other information that the publisher wants. That information is generally specified in the publisher's guidelines. Typical of this information that some publishers want is a short biography of yourself. Guide yourself by looking at a copy of the publication you're submitting to.