Primary sources are the raw materials of history — original documents and objects which were created at the time of the event you are studying. They are different from secondary sources, which are accounts or interpretations of events created by someone without first hand experience.
Primary sources are created:
A primary source is not necessarily your main source!
Finding primary sources in US History is mostly difficult because there are so many that it can be hard to narrow it down to the best one for your project. Here are some great places to start:
These sites have primary sources focusing on the history of California.