How do I begin writing this? I don't know where to start.

When writing my personal statement, I found it easiest to note down all of my ideas in a random order, build upon and refine them, and then stitch them together in a suitable order.
A great place to start is by drawing upon some of your volunteering/shadowing experience. Anecdotal content about what you witnessed and what your learnt from it is always a winner. It is great to incorportate descriptions of any skills you saw displayed by medical staff and explore why these are important in medical practise and, if appropriate, with regard to the key pillars of medical ethics (autonomy, benificence, non-maleficence, justice, confidentiality...). Mirroring these skills with those that you have gained or developed by other means is a great idea.