Refining the thesis: A good thesis does not spring to life from nothing. A good thesis is the product of a discussion you have about your source material and its meaning. Here is what that process might look like:
Put another way, a good thesis sentence will inspire (rather than quiet) other points of view. One might argue that America is violent because of its violent entertainment industry. Or because of the proliferation of guns. Or because of the disintegration of the family. In short, if your thesis is positing something that no one can (or would wish to) argue with, then it's not a good thesis.
More suggestions for developing a good thesis:
You should start thinking about possible theses from the very start of your paper preparation, but you need to examine your primary sources before you can develop a strong thesis. It is impossible to develop a good thesis without already having begun to analyze the primary sources which supply your evidence. How can you know what is even possible to argue if you haven't looked closely at your data?