Use Hughes's poem "Dreams" as model to write your own poem. Begin your poem:
Hold fast to dreams
For if dreams die
Then create your own poetic images expressing how you feel about the loss of a dream.
As previously noted, no one should write this for you. It seems as if that the personalized and subjective nature of the task lends it to being something that has to be done by the writer themselves. This being said, I think that there are some ways you can approach it. One of the striking elements of Hughes' work is that it is filled with images of the loss of dreams. Ask yourself the same element: How has the loss of dreams in your own life or in someone's else's been resembled? I think you should look for images or mental pictures that convey the sensation of losing dreams, of possessing something to see it slip by, or of envisioning something that can be and watching it become something never to be. I think that engaging in some reflection as to what this resembles might be a good place to start in writing such a work. It should be noted that this might turn out to be quite powerful, but could also be quite painful to compose in terms of reflecting on a topic from which others stray. This should not prevent you from composing or working towards a particular end, but one upon which awareness is needed.
As you know, we cannot write your assignment for you, but I can help you. This is a poem by Langston Hughes, so I have moved your question to that group. If you read this short poem, you will see that the author uses similes and metaphors to compare to broken dreams - "a broken winged bird that cannot fly" - "a field frozen with snow" so you must do the same. The poem emphasizes that it is important to have dreams, to not let them go - hold fast to them. You could also write about lost dreams.
Think about a dream that you once had that did not materialize. How did you feel? What images came to mind? If you have not experienced this, then you will have to make something up. Think about something that someone your age would possibly really dream about - getting into a good post-secondary school, having a boyfriend/girlfriend, getting a spiffy new Apple iPad, or it can be something loftier like world peace, the end to the war on terrorism. Once you come up with the basic idea of the unrealized dream, then you can conjure up some images for what it represents to you. Once you get your mind working about dreams, you will come up with some images. Think about dreams and how important it is to have them, whether or not they can always be realized, how you feel when they are not realized, is it worth having them? Are they realistic? Does it matter if they are realistic? Can it be hurtful to have them?
You can make it funny as well, although this poem is not funny -- I'm not sure if this will be OK with your teacher, but you will know this. Also, do you have to follow the rhyme scheme of the poem? You will know this as well.
So for example:
Hold fast to dreams, for if dreams die
Life is a storm cloud high in the sky
Hold fast to dreams, for if dreams fade
Life is a sugarless, tart lemonade
OK, I know this is totally LAME -- but you can do much better. I just wanted to give you an example that you would not be tempted to "borrow" since it is so lame.