What do you want to say about foreshadowing in Othello? As you said, you know what to write, but you need help constructing an appropriate thesis. No doubt, Shakespeare uses foreshadowing to create tension and anticipation of what is to come. Foreshadowing creates suspense and intrigue, and it helps the reader understand later events. If you are to write an essay on how foreshadowing is used in Othello, you could focus on Shakespeare’s purpose for using foreshadowing and provide examples. You could explain how foreshadowing creates intensity and suspense, which keep the reader compelled to read on and on, and foreshadowing helps the reader understand what happens later on in the story and why it happens.

For example, in Act IV, Scene III, Desdemona ironically is singing about her unfortunate end. Desdemona cannot get a certain song out of her head. She tells Emilia about an instance that happened in the past. She shares that her mother’s servant or “maid” named Barbara was in love with a crazy man, a crazy man who did forsake her. She adds that the maid died singing a sad love song:

She was in love, and he she loved proved mad

And did forsake her. She had a song of “willow”;(30)

An old thing 'twas, but it express'd her fortune,

And she died singing it.

Desdemona cannot get this song out of her mind. Desdemona is foreshadowing that her love for Othello will end like the sad love of the maid.

That song tonight

Will not go from my mind...

One thesis that would explain why an author uses foreshadowing would be the following: Shakespeare uses foreshadowing to help the reader predict that Othello will go crazy and Desdemona will die because of her love for a mad man.

Are there instances of foreshadowing that hint that Othello is susceptible to manipulation early on? Did Desdemona know she had married a man capable of savage behavior?

A lot of people have this problem. They get hung up on trying to write an introduction or a thesis statement and it can cause writer's block. I believe you should write what you have on your mind and worry about your thesis statement later. You may find your thesis in what you have written in the body of your essay. At any rate, that's where you should look for it. Your thesis will probably be implicit in everything you write in your essay. 

James Thurber, one of the best American writers, said that he always had a terrible time getting started writing anything. His first drafts always looked horrible. But he believed in getting something down on paper, something tangible. One of his most famous quotes is:

Don't get it right, get it written.

E. B. White says something similar in the last chapter of his valuable book The Elements of Style. 

The word essay derives from the French. It means "an attempt." An essay is always an attempt to express something in words. It rarely comes out the way you want it to--but sometimes you are pleasantly surprised. Writing is a process of thinking and expressing thoughts in words. And then thinking some more and then putting some more thoughts into words. You have to be willing to write more than one draft. You can't make an omelet without breaking a few eggs.

My advice is to write as much as you have on your mind and worry about your thesis later. Your problem is not inventing your thesis but finding your thesis. It is in there. The thesis has to come first--but it doesn't have to be written first.