Moving from Topic to Thesis
It is important to remember that a narrow topic is not the same thing as a thesis statement. Unlike a topic, a thesis makes a claim of fact, provides a claim of value, or makes a recommendation about a topic under consideration. For example, your narrowed topic might be "the underemphasis on foreign language in U.S. secondary schools." A focused thesis statement making a claim about this topic might read, "U.S. secondary schools should require elementary students to take at least one course in a foreign language sometime during the 4th through 6th grades."
Transforming a workable topic into a possible thesis is really just a continuation of the narrowing process, with an emphasis on what you want to say about your topic. In this way, it is much like the "hypothesis" stage of the scientific method. You arrive at a thesis by attempting to make a statement about the topic you have chosen.