First, identify the type of essay you want to write. If you want to provide an explanation of the disease, then you should use definitional or logical models. Divide your paper into parts and dedicate each part to a specific sub-topic.
The more specific the information in your paper, the better. Narrow down what you know, so it will be easier for you. Here are some narrow topics: the effects of the Black Death on religion, the biology behind the disease or the way it spread.
You know that nursery rhyme connected to the plague? You can incorporate it in your essay as a humanist view of the disease. The fact that some parents still sing it to their children and have no clue about its origin is quite disturbing.