First, you need to have someone write it out in beautiful handwriting....or you can use a font on your computer and print it out. Then you need to transfer the poem onto the fabric (check u tube for directions how). then you will need to embroider the fabric (using stem stitch).
Or buy graph paper and a cross stitch alphabet. Plot the lettering down on the graph paper. Using your pattern, stitch it on aida or linen or even weave.
Hope it works out for you. If it were me, I would go to an embroidery shop and hire someone to do it for me.
Good luck with it.