A primary source provides a first-hand record or report of what was done, thought and felt at a particular time and place. These provide the raw material for interpretation and analysis by historians.
We cannot always gain direct access to an immediate, eye-witness account, especially for events hundreds of years ago. So usually we try to get as close as possible, realizing that we are relying on memories, editing, and sometimes even translating.
The first step in finding primary sources in many cases will be to take a look at some secondary sources and encyclopedic sources on your topic. If those sources are written by historians they will include references to useful primary sources.
The materials below provide additional tips for locating primary sources: