First you need to decide in what genre you want your song to be. If your poem is a love song you might be thinking of soft rock or jazz. If your poem is about divorce or a lover leaving you, you might be thinking blues, heavy metal or country. You will probably be most comfortable with the type of music you like to hear in your car on the way home from work, but you should consider different options.
Next you convey to the composer what sort of emotions you want the music to portray. The list of possibilities is long and can include: fear, anger, surprise, anticipation, guilt, depression, pride, jealousy, self-pity, anxiety, resentment, envy, frustration, shame, happiness, hope, enthusiasm, gratitude, grief, patience, sadness, joy, trust, disgust, vulnerability, worry, optimism, denial, shame, awe, disappointment, remorse and contempt.
Take the first two lines of "The Love Song of J. Alfred Prufrock":
" Let us go then, you and I,
When the evening is spread out against the sky."
The music can establish the interpretation of this line to be carefree, angry, or full of anticipation, and you will need to tell your composer how you want the energy of the music to come across.
Next you have to consider what sound effects you want in your song. Your song might call for dogs barking in it or the sound of a waterfall in the background. Almost anything you can think of can be recorded and placed into your song.
When discussing instruments with your composer, you should be as specific as possible. If you want a guitar, you need to tell the composer whether you are thinking acoustic or electric guitar. You want drums in your piece? Saywhat kind of drums you want, conga drums or maybe a drum set?
You want people to say your song has a good beat. The easiest thing for you, as a non-musician, is to find a song that has the beat that you want and share it with the composer. If you're familiar with time signatures then you can tell the composer the time signature for the different parts of your poem.
Finally you want to decide what sort of vocals you want. For opera or classical pieces you can discuss with your composer if you want a countertenor, tenor, baritone, bass-baritone, or bass, if you want a male voice. For a female voice you have the options of soprano, mezzo soprano, or contralto. If you are writing rock, country or alternative music, you may want to give an example of an artist that you imagine singing your song. Also consider if you want your song to be song in a childlike voice, a silly voice, a weird voice, or a raspy voice. When you have worked out d all these elements, you are ready to join the composer in producing the next hit of the century.