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!
Primary sources provide a window into the past—unfiltered access to the record of artistic, social, scientific and political thought and achievement during the specific period under study, produced by people who lived during that period.
Coming into close contact with these unique, often profoundly personal, documents and objects can provides a very real sense of what it was like to be alive during a long-past era.
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 sources focusing on the history of California. Many of them are primary sources but be aware that some are not. Cjeck with your teacher or the librarians if you're not sure.