Where are you from in cali exactly? The first step it to talk to your doctor for sure. Most doctors wont have a problem writing a perscription for you. They will also give you a list of locations to pick some up. I think norml has a list of locations too.

If your doctor wont do it, ask him to refer you to a "specialist". There are certain doctors who specialize in medicinal marijuana, and will write a script a little more freely as long as you have a referal from your normal doctor.