If you need to obtain an ID card, you need to go to the following office: 

Los Angeles County, Department of Public Health
Central Health Center, Medical Marijuana ID Program
241 N. Figueroa St., 1st floor Lobby
Los Angeles, CA 90012

And you must make an appointment between 8 AM  and  4 PM,

If you are caling within LA County, the number to call is:

1-866-621-2204

If you are calling from outside of Los Angeles County, you can currently obtain limited information by calling (213) 240-7785.