How can I write a good thesis statement about foreshadowing in Othello?

This is a great question.  You are not alone in the struggle to write a thesis statement.  It's a tough thing to do.  Based upon your information, I have some suggestions.

First, most teachers give their students some specific guidelines about what must be a part of the written statement.  For example, a teacher may say, "I want you to have the three parts of your argument in your thesis."  Be sure to check what is the case for you.

But at the foundation, a thesis statement "tells it all" in one sentence (I've seen a few teachers want more than one).

Your paper is a literary analysis, so here is my suggestion about how you can construct a literary analysis thesis.

Literary Analysis Thesis

• Must include (1) author, (2) work, (3) argument (the point your essay will prove)


• Your teacher will give you a broad topic prompt for each literary essay (such as, “Irving and Romanticism”), and you will narrow the topic to form a clear, comprehensive thesis including author, work, and argument.


• 
  Example: “Washington Irving’s “Rip Van Winkle” clearly demonstrates the major tenets of American Romanticism.”

Here is the idea: 

The writer (use the author’s name) employs X (articulate techniques and strategies) to reveal Y (the claim you will prove in your paper).

Here are some templates you could use:

In [Title of Work], [Author] uses [Literary Element] to show/reveal [your claim].

EXAMPLE: In "Young Goodman Brown," Nathaniel Hawthorne uses symbolism to reveal the theme of moral ambiguity.

OR:

In [Author's Name Possessive] [Title of Work], [Literary Element] shows/reveals [claim].

EXAMPLE: In "Julius Caesar," William Shakespeare uses characterization to demonstrate that violence is a justifiable means to any end.