An argument essay is one which explores a controversial topic and takes a particular standpoint. It is important to pick a topic which has two conflicting points of view and one which you are passionate about. Read background information about the topic and find enough information for and against the topic in order to make sound arguments about your position.
How is an argumentative essay different from an expository essay?
An expository essay merely describes the topic and provides information. An argument essay examines the pros and cons of a debatable issue and provides sound evidence to support your claim. So it is necessary that the information you gather is current, detailed, and accurate.
Your initial paragraph should give a brief description of the topic, state why the topic is important, and must present your thesis statement. The body of the essay should discuss the pros and cons with supporting evidence for each. The conclusion must restate your claim and say why you think your standpoint is correct based on the evidence you have gathered.
Fore more information on the argument essay consult Purdue's Online Writing Lab