Theme statements should describe an argument, or the author's stance on a certain topic presented in the novel.
A theme statement regarding the topic of communication in Speak might be: "The theme of nonverbal communication throughout the Laurie Halse Anderson's novel Speak is a metaphor for the main character's secret, which she is never able to disclose aloud in the novel."
Another theme statement might discuss the theme of depression in Speak. For instance: "Author Laurie Halse Anderson explores several ways that teenagers deal with depression in the novel Speak, including self-expression through art, self-mutilation or cutting, and various types of friendships."
In any theme statement, make sure you explain clearly what you believe the author's point of view to be, as well as showing what the "topic" is, and phrasing the statement as an argument that can be proven.