You need to check the guidelines from the university in question to see how long it should be (never exceed the maximum) and what kind of information they want. In my experience, the personal statement gives you the freedom to explain why you are interested in the course in question. I would include details of past achievements which are relevant to this course; how you got interested in this subject; what your career dreams are and how this course will help you realise them; and how this course can help you make a positive contribution to society. It's an important document so I recommend you ask a native English-speaker to check and correct it for you.