Brainstorm what you want to write about. Decide where you want the setting to be and who the characters are. Try to use a setting that you are familiar with and know a lot about, such as a childhood home, a school, a restaurant, a patch of woods or a specific street. Try also to limit yourself to one or two characters to keep it simpler; characters can be based on yourself or someone you know, or completely fictionalized.
Write down specific details about your setting and character(s). For the setting, think about the way it smells, sounds, feels, tastes, and looks through the character's eyes. Try to include details from all five senses. Do the same for the character(s). Think about their unusual tastes or tendencies and how they might relate to the setting.
Write down ideas for conflict and events in the poem. A few possibilities might be a disagreement between two characters, an endangered character, a character who makes a mistake, or an internal, emotional issue that the character is thinking about. Keep in mind the setting and the character(s) when deciding what you think the most convincing event or conflict in the poem will be.
Break your ideas up into four to six different sections. You might have a description of the character(s), a description of the setting, what the character(s) is doing in the setting, and the beginning, middle, and end of the event/conflict of the character(s). These different sections will function as the four to six stanzas of the poem, all of which should be four to six lines long.
Begin writing by taking your brainstormed ideas for the character or setting and putting them into line form. Don't worry about rhyming; rhyming often detracts from the rhythm and language of poetry. Instead focus on specific sensory details and images of the setting and characters; this will ensure rich, pleasing language, which is an important part of poetry.
Continue writing stanzas about the event or conflict in the poem. It's okay if it strays from what you originally thought the main conflict would be; often the act of writing leads you to a different, more interesting event or character conflict. Try to go with the flow and write what feels right.
After writing your four to six stanzas, don't be afraid to go back and re-write as much as you want. You won't "mess the poem up"; you can always go back to what you began with. Editing and reworking sentences and lines is an important step to the poem, so make sure you take the time to go back and re-read your work and spend time on anything that you feel is weaker than the rest.