Beowulf is one of the great works of western literature. There is so much that one can say about this work that one will have to be selective. Moreover, one is able to look at Beowulf from different perspectives. For instance, one could look at the theology, social structure, history, assimilation of cultures, literary devices, to name a few. Since I am more of a historian, here is a possible idea. You can study Beowulf to see the merger of Christianity and Roman/Germanic culture. A possible thesis can be: Beowulf displays an uneasy amalgamation of Germanic/Roman ideas and Christian ideals. For the rest of the essay, you can look at passages to see these tensions. For example, the works says:
"Till the monster stirred, that demon, that fiend (Grendel) who haunted the moors, the marshes, and made his home in a hell. Not hell but hell on earth. He was spawned in that slime of Cain, murderous creatures banished by God, punished forever for the crime of Abel's death." Lines 101-108
The very fact the sin of Cain is present show the Christian influence and familiarity with Genesis. The fighting, emphasis on bravery, and the honor seeking in battle shows the Germanic/Roman culture.