Some of the most important work you will do on your paper might be called "mind work." Mind work does not entail any writing (although some people like to keep their ideas recorded by jotting notes on envelopes, napkins, sales slips, etc.), and it starts as soon as you know the assignment for the paper. As you listen to or read the assignment, begin to think about how to approach this subject and what position you might like to take. From the time of the assignment until the time you formally begin to write the paper, spend time with the topic.
Think about the topic at many different times: when you're walking to class, when you're waiting for a bus, when you're in the shower. Just let your thoughts drift around the topic to see what emerges. Treat your thoughts like rough drafts; retrace ideas, reframe arguments, and modify your plans. Talk to friends about your ideas; try out your arguments.
You also can use this time to read academic journal articles about the topic. Reading these articles can give you new perspectives and ideas about arguments you might make. You may find that authors of these pieces already have tried some of the arguments you planned. You even may find an article that gives suggestions about what kinds of arguments need to be made about this topic.
Do not censure yourself as you rummage through thoughts about the topic; give yourself a wide latitude and entertain a variety of ideas. When you find yourself favoring particular ideas or aspects of particular ideas, you have discovered the focus area for your paper. At that point, you are ready to sit down to compose a thesis.
If you find the topic seems to be getting larger as you think about it, take time to study the infrastructure of the idea or collection of ideas. What are the categories of ideas you have gathered? Which of those categories most intrigues you or which do you think best addresses the assignment? Which part of that category might you choose to develop? Determine the perimeters for your idea. Continue to do the "mind work," but remind yourself occasionally that you must do your brainstorming within the perimeters you have selected.