Knowing what type of source you need will also help you find the correct source.
Primary sources are first hand accounts created at the time of an event by the people who directly witnessed or were directly involved. The advantage of using primary sources is that you get your information straight from the source. You don't have worry about misinterpretation and/or researcher bias. Primary sources can be found everywhere, even the Web -- just check to make sure the information is real. Also, keep in mind that primary sources are subject to bias just like any other source. It's very possible to have two primary sources regarding the same event that are completely different from each other because the people involved had differing experiences (i.e. a Confederate soldier’s account of a battle compared to a Union soldier’s).
Example: Civil War Recollections of James Lemuel Clark by James Lemuel Clark.
Example Primary Resources: Photographs, Letters, Diaries, Speeches, Autobiographies, Interviews, Daily Newspaper Articles
Secondary sources are those created after the event by people who weren't directly involved. This includes books and journals written by scholars as well as reference books. Secondary sources may include photographs or other primary sources and can often offer insight and research into the original event.
Example: Civil War Arkansas, 1863 : the Battle for a State written in 2010 by historian Mark K. Christ.
Example Secondary Resources: Journal Articles, Books, Websites, Magazines
Think about all the emails you’ve sent. These are primary sources. Now, how many of those emails are you saving? Is someone else saving your emails? Do you think will still exist in 100 years? Will 2120 computers still be able to read 2020 documents?
This is the problem with primary sources. If someone does not actively collect and save it, it disappears with time. The more important the person (i.e. politician) and the more important the event (i.e. the Civil War), the more likely you are to find primary sources on your topic. Another problem is age. It may be hard to find primary sources for things that happened thousands of years ago because the papers have literally crumbled.
Example: If your professor requires you to use primary sources for your research paper and you pick the obscure Egyptian pharaoh Wadjkare, this would be a very frustrating paper to write. There is only one known resource that appears to list his name and it provides no information about him. Save yourself the frustration and find someone else who has more existing primary sources to choose from.