Writing an argumentative paper means to choose a stance or a side on a topic (usually something that you've decided), and then presenting the reasons why you chose your side.  This is where you'll need to find sources to back up your claim.  Depending on the assignment, you might need to find primary sources or secondary sources.  Examples of primary sources are:

• Letters
• Emails
• Interviews
• Photographs
• Videos  
• Autobiographies