When you are writing a poem, there is so much flexibility. So, you should feel a sigh of relief and be free to be creative.
In light of this, you want to have a theme. This is probably the most important part of a poem. Think of something that you are passionate about or at least something that you want to write about. You can write an poem about your favorite book, if you want. This is to show you that the sky is the limit. Some great poems are about boats, dogs, and so much more.
Second, you should think about the form of the poem. Will it be free verse. If so, this is the easiest. Or do you want a rhyming scheme or something else?
Finally, you need to revise and then revise again. Did you use the right word? How does it sound? Ask your teacher to review it and give you some feedback.