What should I include in my personal statement?

You should include achievements which are relevant to the subject you are applying for and which occurred within the last 2 years. Good examples are summer schools, awards, projects. Another good idea of subject matter to include is books you've read around the subject which has had an impact on you or made you think more about the subject. It's important when you mention books to mention your thoughts on the books, not just list them. When talking about hobbies such as musical instruments it can be useful to say how learning /studying this has changed you, e.g learning saxophone expanded my organisational skills or orchestra helped you explore team working skills.This can be used for school subjects as well, mentioning certain topics you liked.