I already wrote my paper but if my statement isn't solid my teacher hands back the paper In Romeo and Juliet, Shakespeare....
Without knowing what you have already written makes it difficult--if not impossible--to provide a thesis statement that will direct the rest of your essay. But, perhaps, some suggestions can be made if you must rewrite your essay.
If you are considering fate as the power suggested in the Prologue--"star-crossed lovers"--then you can make a statement that contends that fate directs their lives as they are unwillingly caught in the "ancient grudge," unfortunate deaths of loved ones, and untimely events that bring about their deaths.
On the other hand, if you contend that fate does not dictate the actions of Romeo and Juliet, you can write an introduction that includes a famous quote from another play of Shakespeare, Julius Caesar, in which Cassius tells Brutus,
The fault, dear Brutus is not in the stars,
But, in ourselves that we are underlings (1.2.134-135)
Then, the thesis can state that Romeo and Juliet, while often blaming fate, actually create their own destinies by the impetuous choices of actions, their overly emotional responses to events, and their fatalistic attitudes.
Of course, the better that you can support and illustrate the thesis with passages the contentions of your thesis, the stronger your essay will be.