I am a U.S citizen. Do i need a passport and visa when flying from California to Guadalajara. I do not have a passport and I will only be visiting Guadalajara for 4 days on business.
Yes. According to the Mexican Consulate General in San Francisco, as a US citizen you need a passport to enter Mexico:
Effective March 1st 2010, all visitors traveling to Mexico either by air, land or sea are required to present a valid (non-expired) Passport or Travel Document from their country to enter Mexico.
To apply for a passport, you can start by visiting the US State Department's How to Apply for a Passport page.