Two 30 minute interviews, with more general and scientific questions
First interview: general; Second interview: medical
Read about people's experiences online; watch YouTube videos
Prepare as much as you can online
Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
My interview was in early December at the College. I arrived the day before and was offered overnight accomodations at the college. In the morning of the interview I was asked for my ID and told me where to wait. The other interviewees were in the same room, so we could talk. No students from the course/college were available to ask questions. One of the interviewers fetched us when the time for our interview arrived, and led us to a private room where we sat around a table. 2 interviews, each 30 minutes long, each with 2 interviewers. The first one concerned my background, personal statement, why I'm interested in the subject, and so forth. The second one was with two doctors and was purely scientific/medical.
The first interview included questions about my education background, personal statement, volunteering, work experience, why I would like to study medicine, why at Cambridge, etc.
The second interview included questions about the medical world, eg what are the biggest challenges facing medicine in the next 10 years, what's the most important thing about developing new preventative medical measures, etc. It also included questions involving epidemiological graphs and anatomical illustrations that were mostly about reasoning and logic. A question about GCSE-level mendelian genetics was also asked using pedigrees.
Read, mostly at The Student Room, about other people's interviews, and watched a YouTube video by the university about the Cambridge medicine interview.
Private school 1-on-1 Oxbridge tutoring isn't necessary to get into Cambridge. Do extensive research online about other applicants' experiences and what to expect, that's the best way to prepare and to prevent being stressed - knowing what to expect.