Decide the general field you want to write about before you start writing a research paper. You need to do some research before you can write your thesis. Gather evidence. If you are allowed to choose your own topic and you can't think of anything, read the subject headings in the "Library of Congress Subject Headings." Browse through encyclopedias, ask questions, ask for suggestions and ask your professor for help.
Choose a good topic, then a specific issue, and have an accurate knowledge of the issue. Your idea should be original and interesting. A subject is broad and general; a topic is the specific issue being analyzed. The thesis is the specific research query. After you do some research, you might have a working thesis, the main idea. Don't choose overused topics that bore you; you will loose interest. Never choose a topic that is inappropriate or offensive.
Remember, your topic is the specific issue, and the thesis is the particular research question. Focus your topic on a specific research problem. Your thesis can be controversial, but it must be logical. When you choose a thesis topic, study the teacher's guidelines, choose a topic that interests you and then check to make sure there are resources available. A librarian can help with that and show you how to make sure your resources are credible, appropriate and informative.
Create a thesis statement that informs your reader how you will analyze your subject matter. A thesis is an interpretation of the subject, not the subject. Your thesis should be one sentence long. For example, very basically, your subject might be: Americans grow and eat a lot of apples. Your thesis might be: Americans eat a lot of yellow apples because they are sweeter, cost less and are healthier than red apples. And then prove it. Remember, your subject is general, your topic is specific.