First: make sure you thoroughly understand the assignment. Ask yourself:
-- Be sure to consult your instructor If you have any questions.
This video from NCSU shows what goes into picking a topic.
Select a general topic or idea you will be interested in, and write about it engagingly for your readers.
If you are stuck for ideas to develop, browse various sources for inspiration:
As you browse and brainstorm, write down interesting key words and concepts. These words will likely become search terms in your research. They help develop your idea into a focused topic and use research sources effectively.
After finding an initial idea, do background research to learn more about the subject area. This helps you explore the topic and develop the research question you will explore.