Find out what other people have written about them. Write a page, just for yourself, to see if you can articulate some of your ideas. Have an intelligent discussion with your advisor. Try to write a single sentence that states your central problem or idea. If you can't, you might need to refocus your efforts.
Many colleges offer senior or honors thesis options.
Some of these colleges offer tools to assist in the process. This site helps guard against last-minute disasters. It analyzes every step of the process and helps you to be just as rigorous with your own approach. It provides plenty of help with brainstorming, outlines, paragraph construction, and other organizational issues. Many many links as well.