This is a very broad question, and you might want to think about what specific poems you want us to compare. Atwood is certainly a prolific writer, and as a result has published successful books of poetry as well as her famous novels.
I suppose one thing that will help you as you think about which poems you want to compare is to identify some of the prevalent and dominant themes in her work. This will also help give you a sense of how to define or to capture the essence of her poetry. Essentially, much of her poetry encompasses the search for identity and how this is worked out against a backdrop of colonial history with the European settling of the Candian wilderness. Allusions to Biblical and mythological stories abound, and key motifs are metamorphosis and adaptation as the individuals in her poems seek to find an identity for themselves.
This is something that should help you to identify two poems or groups of poems to compare. I have included more links below to help you as you research the work of this fascinating author.
Which poems are you being asked to compare? In what way are you being asked to compare them? For their imagery? For their subject matter?
If you have free reign, you might compare the effects of the first person perspective and imagery used in her two poems, "This is a Photograph of me", and "Morning in the Burned House". Atwood generally has, what I feel is, a sarcastic tone (and I love it!). One of my favorite poems by Atwood, is called "Half-Hanged Mary". If you can do a "comparative study" of ONE of her poems, to another piece of literature or a time period, couple this one with the Salem Witch Trials. It's brilliant!