For some assignments, freewriting maybe a good way to get started. When we freewrite, we're no longer thinking, "This is going to be the finished product, and it must be perfect," but, "This is simply a part of the process." Once you know what your assignment is, sit down at the computer and brainstorm. Follow your ideas wherever they take you. The trick is not to stop. Free associate. Don't worry about spelling, grammar, or structure. Simply write.
After fifteen minutes or so, print out what you've produced and read over it, pen in hand. Circle ideas that seem useful and relevant. Scratch out ideas that don't really have anything to do with your topic. Draw lines among related ideas. Having somewhat narrowed your focus, go back to the computer and freewrite some more, this time using your highlighted ideas as a guide.
After a couple of passes through this freewriting process, you'll probably begin to see what your main idea (your main argument) will be. You'll be ready to craft a thesis.
Writing a paper without a thesis is like driving without a destination (though much less fun). A thesis tells us where we're going. It gives us something specific to accomplish. It also tells our readers what to expect. In short, a thesis makes both writing and reading easier. Once you have a main idea, try stating that idea as specifically as possible in a sentence or two. This is your thesis.