Look at the sources you have for your paper. For each source determine the primary argument the author is trying to make. Ask yourself it it is a valid argument, and why or why not. The answer to the question will help you start developing a structure for your thesis. Remember it is an argumentative paper. No matter what you say, there should naturally be an opposing viewpoint. If there is not, then there is no argument to be made.
Write down your thesis as soon as you have an idea of the argument you would like to make. The whole paper is going to be structured around your thesis statement. Don't worry about it being perfect when you first write it down. The point is to take the first draft of your thesis statement and critique it for improvements as you write your paper. Ask yourself if this statement is taking a clear stance or position regarding an issue. Don't make the statement too vague. Doing so may require you to write a book to adequately cover the points that are necessary to support the statement.
Think about the opposing arguments. The opposition to your thesis statement should have a clear argument against each one of the points you have to support your thesis. When you think about the responses to your arguments, determine how strong each of your points are. If the opposing argument seems stronger, that may not be a point you wish to present in your paper. Otherwise, you will want to search for more information to make your thesis stronger.
Review your paper. Make sure everything presented in the paper is clear and ties back to your thesis statement. If there are any clarity issues, rewrite the unclear passage or take it out altogether. A strong thesis paper stays on track with the main focus point, which is your thesis statement. The conclusion should bring everything back together, while not presenting any new information. Explain the importance of the issue, and why the points presented in your paper are the most relevant to supporting your thesis statement.