Be knowledgeable about your topic. For example, if you are researching World War I, you won't find primary sources by typing "WWI" into a search bar, because people didn't use that term at the time.
Sometimes secondary sources can lead you right to a primary source. Example: an article about the Cuban Missile Crisis (secondary) references a presidential speech (primary).
Google can help. Try googling a topic keyword + "papers" or "archive" --you can turn up some great results. For example, you can find primary documents by googling "President Roosevelt" + "papers".
When you are discussing a primary source for a paper remember that unlike secondary sources, YOU have to do your own interpretation and analysis of the document.
How do you know if your primary source is reliable? Stick to items found in library and museum archives.
Any questions? Come ask Prep's Librarian, Mrs. Hermeline.