An easy way to develop your thesis is to make it into a question that your essay will answer. What is the primary question or hypothesis that you are going to go about proving in your paper? For example, your thesis question might be “how does cultural acceptance change the success of treatment for mental illness?” This can then determine what your thesis is - whatever your answer to the question is, is your thesis statement.
Although it is tempting to simply read over your essay and use the spell-check tool, editing your paper should be a bit more in-depth. Have at least one, but preferably two or more, person/people look over your essay. Have them edit for basic grammatical and spelling errors as well as the persuasiveness of your essay and the flow and form of your paper.
Whenever possible, choose a topic that you feel passionate about. Writing about something you enjoy certainly shows in the final product, making it more likely that you will be successful writing a paper about something you enjoy.
If you find one really awesome book or journal that fits your topic perfectly, try looking in the works cited/bibliography/reference list at the end of it. This should contain many more books and journals that are about your topic as well.