"Peer-reviewed" articles, also known as "scholarly" articles, are those that have been through a thorough review process before being published. At this level of coursework, it is expected that the articles being cited in your papers will be peer-reviewed. Most databases offer the option of limiting search results to such articles. Take advantage of this option whenever possible.
After selecting an issue that interests you, it will be necessary to obtain background information on the subject. This material is best found in journal articles. Depending on the topic, there are various databases to search. Select a database to begin with, based on the main focus of your policy issue.