Knowing what your assignment is about is the first step in choosing a research topic. Usually, teachers provide an outline listing all the necessary requirements. This outline should be followed closely. It will list the types of resources you are permitted to use as well as how recent your information has to be. Talk to your peers to make sure that you're not missing anything. Talk to your teacher if any part of the assignment is unclear. Start early so that you will have enough time to gather all needed materials.
Make a list of topics that interest you and that you would like to learn more about. Your topics should be ones that you feel you will be able to explain and that you think your teacher and peers will also find interesting. Pick ideas that you can easily find information about. Type key words on the Internet and see what sorts of information and journal articles come up--this might trigger ideas on what your paper should be about. Textbooks, newspapers, and class notes, are also useful when looking for topics.
When you are initially searching for information on your possible topics, research widely and gather as much background information and evidence as possible. Writing down a set of useful keywords for each topic will make the search faster and easier. Find information on as many topics as you can, not just the ones you prefer. However, the amount of information you gather should be manageable--research widely but not too broadly. Once you have basic information on most of your topics, you can narrow down your search to the ones that will fit your requirements.
A thesis statement is a question or idea that you hope to prove with your essay. Write your thesis statement in question form. If you have sufficient information to answer that question concisely, and can outline your paper from your introduction to your conclusion, then you have your topic. If not, research any other topics on your list until you find one that is a good match.