There is a multitude of data available on the web.  Here are some useful resources for GIS data and remote sensing images.  Some data are free, some are not.  In either case you can learn about the types of data commonly used in GIS applications.

• SANDAG  San Diego Association of Governments
• U.S. Fish and Wildlife (California)
• ESRI and the Geography Network
• U.S. Census Bureau
• EROS Data Center (USGS) - Remote Sensing Imagery

Try searching for "free GIS data" to see what else is available.