Figure out what the theme is that needs to be included in the thesis. In many cases, you will be deciding on the theme of a literary work. For a thesis statement, you need to focus on one theme only.
Decide what you want to say about the theme. In some cases, your thesis statement might simply be proving that the theme exists within the work. However, you can say more. For example, you might be proving why the author chose to include a particular theme or what significance the theme held for the characters or plot.
Turn the information you've gathered so far into an arguable thesis statement. A thesis statement needs to be something that can be argued, not a fact. An example of a thesis statement involving a theme found in a literary work might say something like, "The author uses the theme of destiny and the characters of the novel to demonstrate what little power people in general have over their own lives and the decisions they make."
Make your thesis more specific as it relates to the theme and the literary work you're writing about. For example, using the example above, you can add more details about the work to make the thesis statement narrower: "The author uses the theme of destiny to demonstrate how little control the character Damien has over his life, particularly in regards to his relationship with Vanessa, thus demonstrating all people's lack of power over their own lives and the decisions they make."