Before you start browsing for ideas, check the assignment requirements carefully to make sure your topic satisfies your instructor's requirements.
All these will affect your choice of topic. For example, if your instructor wants you to write a 10-page argumentative paper related to a historical event in the United States, choose a historical event that is controversial or has aroused debates. Make sure it happened in the United States, and that there is enough information for you to spread it out into 10 pages.
You can find topic ideas from different sources, such as:
Looking through the table of contents of your textbook or a book related to your course can give you some ideas for a paper. You can also use databases -- SIRS and CQ Researcher each have collections of topics that are great for finding ideas (check out the linked videos). Watch the video below for help on narrowing down a topic: