A good thesis statement should be clear and to the point. It should be the last sentence of your introductory paragraph, and it should clarify what the essay will be about.
It should NOT EVER be informal, address the reader or include any statement that represents telling the reader instead of showing (i.e.: "in this essay I will..."). Don't tell the reader what you will do; do it!
It should also NOT include any information that will not be covered in the essay or piece.
An example of a thesis statement on the controversial topic of legalization of marijuana might be: Marijuana should be legalized because legalization would lessen the crime rates attached to marijuana usage, decrease the number of people who go to jail for the offense, and stimulate our economy.
Based on that thesis statement, the reader can infer that the rest of the essay will expand on the benefits mentioned, in the same order they were mentioned.