I would also recommend using an existing sonnet as a template. With my students, I have them write a sonnet in the style of Sonnet 130. Because it starts off so seemingly insulting, my students seem better able to sink their ink into the task.
I have them follow the form and style, and try for iambic pentameter. If that's too hard, just count out ten beats a line. Then use a similar rhyme scheme (this one is ababcdcdefefgg). The main point is in the rhyming couplet at the end, and it is a reversal of the original point--she may not meet any of the conventions of beauty, but she's rare and she's real.
SONNET #130 By William Shakespeare My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red;
If snow be white, why then her breasts are dun;
If hairs be wires, black wires grow on her head.
I have seen roses demasked, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound;
I grant I never saw a goddess go;
My mistress, when she walks, treads on the ground.
And yet, by heaven, I think my love as rare
As any she belied with false compare.
You can keep them theme the same and change the comparisons but keep much of the original structure. Be creative and have fun.
I think the best place to start would actually be to take a look at some of Shakespeare's Sonnets. They are beautifully written. You might also take a look at the text itself, especially the balcony scene. You can't go wrong modeling your work after Shakespeare himself. Hope this helps. Brenda
Many poets, including Mary Oliver who has written many books on how to write poetry, suggests a very good way to begin is to choose a poem you like and then imitate it. First do some basic analysis: what is the rhyme scheme (if any)? How many words are there in each line? Does the poem use metaphors and similes? Are there any symbols? What sort of detail helps make the images clear. Detail makes a poem: “red” says little for everyone has a different color in mind with “red”; “tomato” is a much more lively and specific word to invoke that color. There are many different kinds of poems: narrative poems that tell a story, lyric poems that describe an emotion, haiku, sonnet, ode, and many more. Does your teacher want you to experiment with a variety of these? Make use of rhyming dictionaries such as http://www.rhymezone.com/ . For guidelines on writing poetry also see http://pbskids.org/arthur/games/poetry/what.html and http://www.wikihow.com/Write-a-Poem. Good luck!