A typical essay assignment in high school or first year university literature classes is explication of a poem. Often, the main purpose of the assignment is so that the teacher can see if you understand the literal meaning of a poem despite its use of unfamiliar or archaic poetic language.
Explication in many ways grows out of the exercise of glossing in ancient grammar, and works on a careful word-by-word level.
The introduction of your essay should contain a general summary of the poem, including the name of the author, title of poem, date it was first published, length, and genre. Next, you should identify any general features that apply to the entire poem such as narrative voice and tone.
The main body of the essay should be a line-by-line or verse by versed summary restating the essential meanings in your own words, and explaining any proper names, allusions, unusual words or usages, and complex syntax.