Its very important that we have a good thesis statement when we write an essay, this is the reason of why I want to show you a couple links that will help you developing a good thesis. The firts link show us the importance of the type of essay we are going to do and whow it affects the thesis: http://owl.english.purdue.edu/owl/resource/545/01/
The second has more information and will help us on how to write a good thesis, which requires some excersise: http://www.unc.edu/depts/wcweb/handouts/thesis.html