Market guides will often tell you whether a cover letter is required, preferred, or not required. In the first two cases, you will need to craft a professional cover letter.
Publishing is a business, so your letter should follow standard business format. Place the date at the top, space down, and then type the name of the editor and the address of the magazine to which you are submitting. Greet the editor with Dear Mr. or Ms. (If you don’t know the gender of the editor, use the full name—it’s better not to guess wrong!) In the text of your letter, mention something positive about the magazine, but don’t be overzealous or unbelievable in your praise. If you have never seen the magazine, don’t reveal that fact. Editors don’t want to know you haven’t bothered to read their publication. You might instead compliment the publication’s website or unique title.
Go on to indicate the number and title of the poems you are submitting, and briefly mention why you think they are appropriate for the publication in question. You may wish to include a short two sentence biography. List your most relevant publishing credits, but this is not time to include a full curriculum vitae. If you’ve never been published, just skip this part of the cover letter. Don’t make a point of your lack of experience by writing, “I’ve never been published, but I hope to make it into your magazine.”
Your cover letter should not exceed one page. Editors may note in their submission guidelines any other additional information they wish to see included in the cover letter.