One way to help students generate ideas for their poems is to have them create lists. Choose a topic and time students as they write down as many single words as they can think of relating to your chosen topic. You might ask students to list everything they've done that day, all of the things they see on their way to and from school or items they can see in the classroom. Once students have a list, you might ask them to choose a number of items to write about. You can work with them on developing descriptions of the items they have listed to develop their poems.
Another way to start developing ideas for a poem is to brainstorm words and phrases that describe a sensory experience. You might take your students on a walk and ask them to make a list of all the colors and textures they see. Ask students to describe the sounds they hear in the classroom. You could set up stations around the classroom with different textured objects, have students visit (without seeing the objects) each station and describe what they feel. When students have developed a description, have them turn the description into a poem.
Ask students to remember a recent dream that was silly or funny. Have the students brainstorm ideas about the memory of their dream, paying attention to the setting and any people or objects that were out of place. You can also give students a fantasy situation -- something that would not occur and is grade appropriate. Younger students might be introduced to a world where a dragon is exploring an amusement park. Older students might be introduced to a world where they are some of the first humans to inhabit Mars. Once the situation is presented to students, have them brainstorm what they see, hear, smell and touch.
Another way to introduce writing poems is to present a classic or contemporary poem. After a discussion of the topic, theme and structure of the poem, ask students to think of a few ideas for similar topics they might write a poem about. Have your students model their poem after the poem you presented using their own subject and descriptions.