Which main points should you aim to get across in your Personal Statement?

Why you wish to study the subject you are applying for and why you think you are right for that course 

Any activities you have pursued outside of the syllabus and why these could be relevant to your course and University life 

what skills have you obtained throughout your life which you could transfer to your course. 

Demonstrate in anyway that you're a critical thinker e.g. have you taken the EPQ, written any dissertations or read any scientific journals etc. Write whatever you can to demonstrate this relating to your course. 

outline your long term plan 

Finally, don't panic! if you can't get started right away remember that that is normal and experienced by a lot of students. Focus on highlighting your strengths and love for your subject. Don't worry about the word limit too, you can cut it down later!