An excellent way to do this is look at the word choice, the figurative language and the imagery. Those are some of the primary devices that poets use to create tone and mood.
For example, does he use words that are positive or negative?
Does he use similies and metaphors to compare to positive or negative things?
Take a look at the pictures created throughout the poem--are they positive or negative?
Try to be as specific as you can next--positive and negative are just for starters!
If it's positive, use words like joyful or celebratory.
If it's negative, is it angry? sad? confused?
The more specific the better!
The best way to do this is to get out several of his poems, and a piece of paper. Write down the title of one poem, and as you are reading it, jot down different emotions and feelings that you have as you read the poem, or different things that you think about as you read it. Do this for all of the poems. Then, look back over what you wrote down. Are there any repeats? Do you write down "happy" more than once, for example? If you find several emotions that are repeated, or that are similar enough (like sad and lonely), then use those for the foundation of your paper. I would pick maybe three of those emotions.
Next, I would fashion an essay based around those three emotions. Start your first paragraph out by introducing the poet, mentioning the poems, and then putting your three emotions into one sentence at the end of the first paragraph (which will be your thesis sentence). For example, your thesis sentence could be something to the effect of, "The poetry of John Keats elicits the emotions of happiness, sadness, and awe." (Fill in the blanks with whatever emotions you came up with.)
For the body paragraphs of your essay, focus one or two paragraphs on each emotion, then give examples from the poems that made you feel that emotion. For example, "Reading this poem (insert poem title) made me feel happy when (insert a line or two of that poem) because (insert why)". Do this for all three emotions, and go into as much detail as you need to get the length you are required to write.
Wrap it up with a concluding paragraph where you restate your thesis, summarize any main points, and tie it all together.
I hope this helps. In addition, I have provided links below that will lead you to some eNotes sites that give further advice on writing essays. Good luck!