In general, a prospectus must:

• Define the general topic/question/issue.
• Review how others have handled that topic/question/issue (i.e. the existing scholarship).
• State how what you propose to do is different and significant. It may be that while the topic has been discussed, no one has asked your particular question (which, according to you, needs to be asked because...). It may also be that you are testing or adding to the accepted wisdom on the topic. Whatever the case, you need to establish how you are doing something different from what has already been done.
• Describe the materials you will examine, and argue why examining those materials is appropriate to your purpose. Since you don't yet know what those materials will show, you have to make a case why you think they will provide you with what you want to know.
• Tell how you are going to approach/analyze the materials. Are you going to read/examine them from a particular perspective (i.e. race, class, gender) or employ a specific form of analysis (i.e. quantitative, semiotic, content analysis, etc.)? Explain why the approach is appropriate to your purpose.
• Describe how you will organize your thesis. While there is no required format, the standard for an essay is an introduction, three chapters, and a conclusion. You don't need to state in the prospectus what your introduction and conclusion will be, but you should attempt to define the questions and issues that each part of the thesis will address. 
• Students choosing to produce an honors thesis in an alternative form—for example, a website or an exhibit or a scholarly article accompanied by a reflective essay—should consult with their thesis advisor and reader about appropriate formal criteria.
• Provide a secondary and primary bibliography.

In all this, it is important to remember that the prospectus is a plan and not an ironclad contract. In the course of actually researching and writing your thesis, you will depart from it. However, the better you devise the plan, the more it will help you.