Hmm. How can I, or what would I argue? If I were seeking to create a thesis statement for an essay about this poem, I would start by reading and rereading the poem, to make sure I had a strong understanding of it. I would use related tools, like the explanations available in the enotes study guide (see the themes section: http://www.enotes.com/beowulf-wilbur/24770 or the style section, for example: http://www.enotes.com/beowulf-wilbur/24771), to expand my understanding.
Then, I would start sketching out loose ideas. Where they would go would depend in part on how big the paper was and what limits I had. May I, for example, compare it to the epic Beowulf? To other twentieth century poems, such as Auden's "The Shield of Achilles"?
If I had to make an argument just about this poem, I would look at the relationship between style and content. Specifically, I would argue that the style fits the themes, and that the meaning of the poem can't really be grasped without discussing both.