A thesis statement is a declarative sentence that asserts the position a paper will be taking. This statement should be both specific and arguable. Generally, the thesis statement will be placed at the end of the first paragraph of your paper. The remainder of your paper will support this thesis.
After you've landed on a satisfactory topic, your next step will be to solidify the position you would like to take and write a clear and succinct thesis statement which will lay the foundation for the rest of your paper.
For the sake of example, let's say that you've chosen to argue the merits of eating locally grown foods. You want to focus on the positive effects that this will have on one's health, the local economy, and on global ecology. You also want to dispel the myth that eating locally is more expensive, and therefore, the exclusive purview of the upper middle class.
An example of a thesis statement outlining your position might look like this:
The locavore movement that has gained popularity in the United States over the past several years offers a way to increase health, support the local economy, and promote global ecology by making some simple changes to the way that you and your family eat. Although frequently criticized for being far more expensive than eating factory-farmed foods, the truth is that the costs of home gardening and the prices for which you can purchase food at your local farmer's market are often far less expensive alternatives than buying from a chain grocer, not to mention safer and more nutritious.