Write an introduction to your topic. Depending on the setting, this will take different forms. A Model United Nations position paper, for example, would include an introduction to the country you represent and its history concerning the topic under debate. For some debate tournaments, the topic will be a specific resolution under debate. One example is "Religion is to blame for extremist violence." In this case, your introduction should consider the topic in a general way, including its general significance and the history of debate surrounding the topic.
State your position (or your team's position) regarding the topic. A clear, well-formulated thesis statement will help guide the evidence and tactics you use to debate. In the case of a proposed resolution, use a "because" clause to formulate a thesis for or against the resolution. For example, when arguing in favor of the resolution "Religion is to blame for extremist violence," use the resolution itself to make the thesis statement, "Religion is to blame for extremist violence because it requires uncritical obedience to those in authority."
Cite evidence in support of your arguments. In general, the more evidence you prepare, the better -- but know which evidence is more important in case you have to cut out some examples due to time constraints. Include bibliographic data for your information.
Comment on the significance of the topic and your position. Through the course of a debate, less important ideas take a back seat to more weighty issues as debaters hone in on the issues which divide them. Think ahead of time about what issues might emerge as more pressing or important so you'll be better prepared to comment on them.