That's a little too vague but let me take a stab at it anyway

I'm assuming that you mean your primary doctor said or wrote on a piece of paper that you might be a candidate for MMJ. Then if that is so all you need to do is go to a doctor that does MMJ prescriptions and pay for the certification.
Depending on where you are in Cali will depend on the cost, check local magazines like The Reader here in San Diego and there will be 3 or 4 pages of ads for doctors and dispensaries.
Hope this helped, peace out