As you can see, the question requires breadth (3-4 poems) and depth (it wants you to refer closely). How should you start? Begin with the basics: quickly list 3-4 poems (making sure they're written by 2-3 poets) which you like. Don't worry too much about the choosing because a topic like this will give you plenty to say - just make sure you feel comfortable with the texts. Once you've chosen, make a short plan. This will give your essay direction and ensure you bring out the contrasts - the 'compare' aspect of the question. The factors you can consider at this stage are: the pronoun the speaker uses (I, we etc.); who the addressee is (a group, a lover, a stranger); the certainty of the voice (does the persona/ speaker break itself down through ambiguities or is the confidence of the voice in some way a bit too much to be convincing?); how sympathetic the character is (perhaps you are supposed to dislike them!); and where the author is in all of this. Choose some themes that give you a good contrast and find which words and literary techniques create the impressions you're getting. Try and end with pushing the contrast a bit further by asking why you saw these differences.