How should I structure a personal statement?

The basic structure of a personal statement is roughly 5 paragraphs with 80% of the statement focussed on academics and 20% on extra-curricular, with this 20% normally covered in the final paragraph. For the academically focussed part of the statement, the key is to demostrate depth of understanding rather than breadth. This typically means a fairly narrow topic per paragraph stating and embed your own opinion with justification into the topic - this could be a synopsis of your EPQ, an essay you have written, or a current event. 

For the sentence structure of your personal statement it is important to remember you are writing a piece of English - an admissions officer has to read hundreds of these so make yours interesting and memorable. The traditional pitfalls to avoid here are cliched statements ("I am passionate about economics") and lists ("I read X. I have also read Y. I then did Z"). In general, write about things you are genuinely interested in and you'll portray yourself well.