There are two brainstorming or idea-generating techniques you may want to try if you still don't know what you'll write about:

Freewriting

In freewriting, you write anything that comes into your mind when you think about your topic.  The key to freewriting is not to edit or monitor yourself. Write whatever comes to you without a concern for grammar, spelling, or format. Let your mind explore widely and deeply without your inner editor saying, "That's not right!" Everything is right in this exercise. When you think you have written all that you can, write for 5 more minutes. The most interesting ideas often come when you run out of the obvious ideas.

Concept Mapping

Another brainstorming activity is called a concept map. Concept maps illustrate the relationships between pieces of information in an interconnected way. In a concept map, you should place your main idea in the center or top of a piece of paper (or use one of the many software programs available). Then, add connected ideas around it. You can use  different colors to indicate different ideas, or include drawings, photographs, or any other media that helps you build your ideas.