The statement of argument for an essay, paper or other document is most often called a thesis statement. It is the sentence in which a writer lets the reader know the central idea of a paper is and how it will be presented. A well-wrought thesis statement can also give a glimpse into the overall organizational structure of the paper.
A thesis statement must be debatable. It is not an effective start to a persuasive piece if readers could not reasonably have differing opinions. The thesis also needs a narrow focus, as it must be supported with evidence to your claim. If it is too broad, it will be difficult to provide adequate support.
There are a few general claims which most arguments fall under: claims of fact or definition, claims about value, claims about solutions or policies or claims about cause and effect. To determine which type of claim you will be making, you might imagine where your intended audience will stand on your subject and narrow down where you think the biggest differences in opinion will arise.