There are several topics you should cover in an essay analysing a poem. The first is prosodic structure. What is the basic metrical structure of the poetic lines (if the poem is not in free verse)? One the level of line, you need to specify type of foot (iambic, dactylic, etc.) and number of feet per line (tetrameter, pentameter, hexameter, etc.) if the poem has a regular metrical structure. On the stanzaic level, you need to use the standard terms to describe the large scale rhyme scheme of the poems. In some cases, this will involve a scheme repeated over multiple stanzss (terza rima, heroic couplet, open quatrain) and in other a unique overall patterning (Italian or English sonnet) or pattern of repitition of lines (rondeau, villanelle).
Next you need to note the type or genre of poem -- is it comic? epic? mock epic? narrative? -- and which specific period and school it falls into (metaphysical? imagist? symbolist? modernist?) Next you need to consider the major topics covered in the poem and its dominant mood. Also consider the narrative voice of the poem and its relationship to the subject matter.
Theme: This is what the poem is all about. The theme of the poem is the central idea that the poet wants to convey. It can be a story, or a thought, or a description of something or someone – anything which is what the poem is all about.
structure,; it just means how it is set out on the page! For example how many stanzas it has. If they all have the same number of lines and is their a pattern in the lenght of the lines. In some poems stanzas will all be regular but perhaps two lines will be set out on their own between stanzas or at the end. Usually this is to draw attention to what ever is in those two lines. Very often the structure of the poem will reflect he content.when analysing the structure of a poem look at what it looks like on the page and ask your self why. Is it structured in the tradion of when it was written eg sonnet? Or is it structured to reflect the content and if there is obvious quirks in the structure the poet is drawing out attention to something! look out for refrains or ending in rhyming couplets these are to reinforce an idea! I hope this is helpful in some way. Don't panic about poetry, poets write the same as song writers, for the ear so this often dictates structure.