How do I begin writing my personal statement?

The best thing you can do is to start by writing a list, or mindmap, or whatever helps you organise your thoughts. Divide your thoughts into sections - firstly, your academic achievements. Secondly, your personal achievements; sporting, music, artistic. Thirdly - your personal attributes. And fourthly - what it is about your subject that you love, and any particular interests you have within it. Then think about that section - spend some time writing down what it is about it that you love, and any particular extra-curricular things you've done to further your interests in it. Use this as your first paragraph; this is why you are going to university. Once you have this, try to bring the personal attributes, academic achievements, and extra-curricular virtues, within the circle of the subject, and then in association with your potential as a student. Perhaps your dedication to hockey has taught you the value of teamwork, or how to aim for higher than you think you can achieve, but don't be clichéd; tell them why you are unique, because of your commitment. Be confident in yourself; be succinct, but don't be embarrassed about your achievements. You are trying to make the university see how valuable and rounded you would be as a student.