Hi
Firstly - it doesn't have to medical - it can be anything but its how you show its relevance to medicine - for example - working with the public / different age groups / vulnerable people / being part of a team / leadership / resilience etc. You might get these skills from a part time job. Sports team / extra curricular activities can show these skills too (like DoE)
You could join something like Red Cross or St John's Ambulance and become a first aider.
If you do want to try your more closely related to health - my YP found that surgeries / trusts couldn't accept her until she was over 16/17 years, so timing might be a consideration. Try local GP surgery - but one very local to you may have Data / Confidentiality concerns. Try Allied Professions - like a Physician Associate / Physio / Radiology. My YP also approached our local hospital & they had someone who organised for them...so a question of trying to get in touch with the right person and a bit of perseverance. (contact the departments you're interested in by phone & asking who is the right person to talk to & then making contact with that individual by phone / email)
Hope that helps