I would have spent a whole session on this if I had had it. What follows is what I would tell a student of mine if they asked me, over coffee, how to begin.
Dear Editor,
I am pleased to submit my [poem / story / essay], “[Title],” for your consideration in [Magazine Name]. The piece is [word count] words. It is a simultaneous submission; I will notify you immediately if it is accepted elsewhere.
My work has appeared in [one or two journals if applicable, otherwise omit]. I live in [city] and teach English literature.
Thank you for your time and for the work the magazine does.
With best wishes,
Your Name
I have rounded these from editors’ interviews, journal annual reports, AWP panels, and the trade press over many years. They are conservative.
What this means: rejection is the average outcome of any individual submission and it is not a verdict on the work. A poem rejected by fifteen journals and accepted by the sixteenth is not less good. It is the same poem.
Memoir lives in two markets: the personal essay (1,500–5,000 words) and the book-length memoir (60,000–90,000 words).
If you have never submitted before, here is the map I would draw on a napkin for you.