Make sure you understand the assignment. Your instructor may allow you to select any topic you like, or they may require you to select a specific topic within a more general concept. Read your assignment sheet thoroughly before you begin your research.
Avoid picking a topic that's too broad...or too narrow. Your instructor will probably set limitations for the length of your paper. If you pick a topic that's too general, you won't be able to adequately cover it in a few pages. If you select a topic that's too narrow, you may have trouble finding sources. This will prove frustrating for you and will make the assignment harder than it needs to be.
Write about something that interests you. If you find a topic interesting, the research and writing process will be more enjoyable for you. This will come through in your paper and it'll be more enjoyable for your reader as well.