The main purpose of a thesis statement is to set out one's argument right at the very beginning of an essay, dissertation, or research paper. To that end, it must give the reader some idea of what to expect by accurately summarizing the contents of the work to follow. The precise thesis statement will, of course, depend on the work involved, but whatever the subject, it's generally considered a good idea to write a powerful statement that will grab the reader's attention.
In the case of "The Tell-Tale Heart," such a thesis could be as follows:
"Like much of Poe's fiction, 'The Tell-Tale Heart' is told by an unreliable narrator. This forces the reader to draw their own conclusion about the truthfulness of the narrator instead of taking the narrator's words at face value, as readers often do in fiction."
The main strength of this thesis statement is that it allows you to explore this particular work in depth—especially in relation to the unreliability of the narrator—as well as relate it to Poe's other works and to fiction in general. Such a broad scope allows you to draw on many different secondary sources in writing your dissertation, thus giving it much greater depth and originality.