While there are many types of poems, poetry itself has no rules. This means your 20 to 40 line poem can be about anything you would like, whether to express your feelings, commemorate an experience or honor another person. Even though poetry has no rules, your poem should still be focused on the experience and emotions you would like your reader to feel or understand when reading the poem. Methods for writing poetry work differently for different people, so experiment until you find what works best for you.
Brainstorm ideas you might like to write about in your poem. You don't have to have a set topic when you decide to write the poem. Instead, write down events and feelings most pervasive that you might potentially write about. Some thoughts include writing about the day, the weather, how you felt during an important event in your life or a person who is special to you.
Decide which topic from the list you will write your poem about. Begin another brainstorming session, this time writing down adjectives to describe the event, the feeling or the person you've chosen to write about.
Determine whether you'd like your poem to rhyme. Many "traditional" poems are written in rhyming couplets, with two lines rhyming while the next line does not rhyme and instead rhymes with the line beneath it. However, poetry does not need to be confining, so experiment with different styles.
Begin writing your poem. Write a full first draft of at least 20 lines before you go back and look over it.
Read your poem aloud and see if it accomplishes what you would like it to. If it does not, edit your poem or rewrite it until you are satisfied. You can experiment with writing the same poem in two or three different ways to see which way you like best.
Try not to judge yourself as you write your poem, especially if you are a poetry novice. You cannot expect your first poem to be of Shakespearean quality, so take it easy on yourself.