State how this topic relates to the course. For example, if the title of your course is "Feminism in the 20th Century," you might tell the professor that you're interested in writing about the role universities and large corporations played in the advancement of women in the academic and business worlds during the last part of the twentieth century. Include which concepts you learned in class that you plan to apply to this topic.
Discuss your methodology for the topic. Using the example above, if you plan to use interviews with local female businessowners who paved the way for female entrepreneurs in your city fifty years ago or former female university students from that era, mention this in the proposal. Or if you're going to use research from current scholarly journal articles on this issue and analyze the authors' views on the topic, discuss this.
Mention why your topic is relevant. For example, if you're writing about the rise of young, celebrity, African-American entrepreneurs over the past decade -- such as Sean "Diddy" Combs and Jay-Z -- say that this is a relevant subject because even in the midst of continued racial discrimination and the breakdown of the traditional African-American family, there are still some young African Americans who through hard work and determination managed to achieve the American dream.