The thesis is the central argument in your essay. It is a claim that will be developed and supported in the body paragraphs rather than a topic or theme. In fact, essay questions usually offer a broad topic from which you must develop your position or your argument. Your thesis must be something that might initiate debate and need to be proven and something which can be reasonably sustained by the scope of your essay.
Consider the following thesis:
Emily Brontë's Wuthering Heights explores the consequences of class distinctions.
This thesis describes the text in question. It does not take a position or offer an interpretation of the text.
Let’s try again:
Through the juxtaposition of Thrushcross Grange and Wuthering Heights, Emily Brontë's novel explores the consequences of class distinctions.
This is better. The statement refers to certain facts and begins to take a stronger position by explaining "how" the text in question works.
One more try:
Through the juxtaposition of Thrushcross Grange and Wuthering Heights, Emily Brontë's novel explores the consequences of class distinctions. While the first generation remains divided along class lines symbolized by the division between the two houses, the second generation challenges and eventually overcomes these divisions by relying on the redemptive qualities of education to unite the families and the two houses.
This thesis answers the three vital questions: what, how, and why.
What is the topic or matter at hand (class and Wuthering Heights);
How will the thesis be discussed (through an examination of the two houses that are symbolic of different classes) and
Why is the argument significant (the argument is significant because it explains how the second generation is able to achieve redemption through education).