There are probably two themes that you can utilize for your essay on Joan of Arc. Finding the events of her life and the military accomplishments are the easy parts and will make up the body of your essay. Identifying themes and how all the events of her life connect to that theme is probably the greatest challenge.
One theme you could explore and discuss in the opening of your essay is about gender roles. Joan of Arc became an important military leader and led her troops in victory. She did this at a time when women were not viewed as warriors, or even leaders. Mentioning her struggle in terms of a gender struggle would not be a bad way to go. Did the fact that she was a women lead to her subsequent death? What proof can you come up with to support this answer? The topic of Joan of Arc lends itself to the discussion of gender roles in society.
Another theme to explore is one of secularism and theology. Joan of Arc was a military leader during her life, but she claimed to have heard messages from God. How did these purported messages lead to her rise and fall? How did her claims of being a prophet impact how she was received back then? How would they be received today? Why was she canonized a saint by the Roman Catholic Church? These are issues that can be great thesis options. Issues of feminism and theology versus secularism are timeless themes that are still debated in the modern world we live in today. Choose one of these themes and discuss it at the beginning and end of your essay, while tying it to events of her life in the body of your writing.