How should I structure a personal statement for medical school?

It depends on where you're applying to! Oxbridge like a focus on biomedical sciences and any research you have done (e.g EPQ) - for these a 40% science 60% "other stuff" split is appropriate. Otherwise, aim for a 30:70 split so you can demonstrate extra-curriculars more. Here's what you should aim to include in your personal statement, by topic:
Science•Specific parts of your A-level course that interest you and demonstrate how you’ve gone “above and beyond” to research this further If you’re doing the EPQ, include some brief details about your chosen topic and research Any books you’ve read related to medicine Lectures, journals, podcasts you read/listen to 
“Other stuff”•Work experience, voluntary work etc (this may sound obvious but a Top tip: relate your experience and skills you’ve learnt to the skills they are looking for e.g https://www.southampton.ac.uk/medicine/undergraduate/courses/bm5_a100.page#entry_requirements%0A see non-academic entry requirements or FAQs ) • Extra-curricular activities - show off! Demonstrate sports, clubs, societies you have done and include keywords that indicate how useful these skills are e.g. teamwork, leadership, communication