Search for "National Parks" as a starting point. At the top of the results list, note the "Research Starter" which gives an overview of the topic. Use the advanced search for additional keywords, such as "United States" or the name of a particular National Park or Monument. Narrow your results in the "advanced search" options or by using the left-hand column of the results list page. For example, you may limit the results to "articles" or "ebooks." You may find a more clearly defined subject/topic. Consider the importance of the publication year. Determine the type of source needed, such as news, academic journals or magazines.