Start here:

Colorado: Department of Public Health and Environment, Colorado Medical Marijuana Registry

On their recommendation form, there is an area for them to put in your info. If they already have a caregiver, you need a change of caregiver form. Then enclose a color copy of your id with theirs and send it in. Make sure to use blue ink and get everything notarized.

All forms are found in the above link. Keep proof of mailing for everything.