What should be in my personal statement?

A personal statement is much like the cover letter of a CV; it's a small piece of introductory writing at the top of a CV that is personalised to the position you are applying for. This means that, for the most part, you want to do two distinct things: demonstrate your skills, and justify why you are interested in the course.
It is important to note my use of the word demonstrate when talking about how you present your skills in your personal statement; it can be easy to slip into a list-like account of your accomplishments and experiences when you are trying to impress your prospective university' admissions staff, but this will not impress them any more than a few of your core skills presented clearly and confidently. At the top universities, many applicants will have experience, so it is more impressive if you can hand-pick skills you have developed during your experience (etc.) that are applicable to the course you're applying for. This way, a reader of your statement will already deem you appropriate for their university before you even need explain why you've chosen their course.Your explanation of your interest in a particular course is largely just a way of them testing your fundamental knowledge of what a course entails. However they do not expect this to be in any detail, as they do not expect you to be an expert in a subject you haven't studied yet).