An effective essay requires a strong thesis statement -- a declaration of the writer's opinion on a given topic or the central message of the essay in one sentence. A thesis must be sound because it is the backbone of an essay. The rest of the essay expands upon and proves the thesis statement.
Form your opinion on the chosen topic into one sentence. The sentence should describe both the topic and your opinion on it. For example, in the thesis statement "Abortion is wrong," "abortion" is the topic and "wrong" is the writer's opinion. Likewise, in "Deceptive advertising can cost consumers not only money but also their health," "deceptive advertising" is the topic and "can cost consumers not only money but also their health" is the writer's opinion. In an informative thesis statement, such as "Classical music can be played by groups of various sizes, ranging from chamber ensembles to full symphony orchestras," "classical music" is the topic and the rest of the sentence describes the writer's approach to it.
Ensure your thesis is debatable and not factual, if you are writing an opinion essay. For example, "Hamlet is a play about revenge" states a fact about the play; it does not present an argument. "Hamlet's insanity leads to his downfall" is an argument.
Remove any unnecessary words to make the thesis as concise as possible. In "Deceptive advertising can cost consumers not only their money and their finances but also their health," "their finances" is redundant, and can be removed without changing the meaning of the statement.
Replace vague or ambiguous words and phrases with specific words. For example, in "Lately, their works are gaining," the reader doesn't know who or what exactly is "gaining" and when, or what they are "gaining." A better thesis reads as follows: "During the past ten years, the works of artists Mary Cassatt and Rosa Bonheur have finally gained widespread critical acclaim."
Include the three major supporting ideas of your essay, if your teacher requires them. The thesis "Rising crime rates, increasingly overcrowded conditions and growing expenses make living comfortably in a modern city difficult" contains not only the argument, "Living comfortably in a city [is] difficult," but the three main reasons why, "Rising crime rates," "increasingly overcrowded conditions" and "growing expenses."