Whenever you have to write an essay on a text and you are not too sure of where to begin, one good place to start is to consider the various themes of that text. Taking one theme and exploring how it is presented in that text would make an excellent essay. I have included some links below to help you with this as you research this text and think about what you may want to write about.
For me, what is most interesting in this text is how some argue tha text is actually an allegory. Those that view the text this way state that the various monsters in the text are thought to stand for the evil in the world. However, I think there is plenty of evidence to suggest that the text presents Grendel and his monstrous mother as being motivated by human instincts and drives rather than being some kind of terrible representation of evil. For example, Grendel's humanity is stressed through his jealousy when he hears sounds of human joy. In the same way the dragon responds to its natural instincts when it chooses to nest on the treasure.
What you could do therefore would be to examine the role of the monsters and to consider whether we are able to treat them allegorically as representing the force of evil. Does the text support or refute such a view? This would make a fascinating essay topic. Hope this helps, and good luck!