You need to go on the official Cambridge Diet website - from there, you need to go onto the Cambridge Diet Counsellor section and there will be an option to find a counsellor near you.
Do this, and then contact a few by phone/email. They'll arrange to meet you, once there they will weigh you and see if you are eligible for the diet and which step you should go on. I contacted my counsellor on a Thursday, and started the diet the next Monday! It's very simple and easy to do, and the counsellors are really great.
You will need to purchase the products from the counsellor, most people meet them every week (i meet mine every Monday), some do it once a month or once every two weeks, but when there you will be weighed to see how you are getting on, then buy enough products to see you through to your next weigh in, etc etc.