The mind map, or clustering, exercise is good for getting your thoughts out on paper without worrying about sentence formation. It works very well for generating a thesis or arguments because you formulate keywords and concepts and begin to see relationships between ideas.
In order to do a mind map, pick a concept or theme (eg, one of the themes of the course that will be on the midterm). Put it in the middle of a blank piece of paper and circle it. Then begin jotting down all the words and phrases that it brings to mind, circling them and connecting them to the center or to each other. Keep doing this until you feel you have all of the concepts you associate with that central word down on the page.
At this point, you can try writing a thesis. Can you see a definition of the term emerging? Can you formulate an argument about what the thing means, using several of the terms? Do you see connections between disparate ideas? Write a statement linking these key words together. Remember that a thesis is an argument, not a topic.