The first step in writing any major paper is to understand the assignment. Read the entire assignment carefully, noting requirements and due dates.
Defining a research question, problem, or issue is the next major step. Since this is a capstone project, review this course and others you have taken. What topic might have been introduced that would lend itself to a more in-depth look? What would you like to know more about? Is there a topic you've encountered in multiple classes that piques your interest?
Do some preliminary research. You can start with a general source like the ones listed below. You want to make sure your topic is broad enough to write a lengthy paper but not so broad that you are overwhelmed with information and find it hard to focus. You want to be able to find what others have said about your problem or issue but leave room for your own hypothesis, based on your research and data gathering. You should also check the library catalog to see what might be available.