Another important role of the PS is that it allows you to explain, although briefly, some information that may not be accurately represented by your transcript or Curriculum Vitae. For example, your GPA might be low because your school places a cap on grades given to students belonging to your major. Another example would be explaining the connection between your undergraduate major and your chosen graduate major, which might seem incongruous at first inspection. For these reasons alone, it is extremely important that you follow a guideline in composing your own PS.
Then look at some resources for writing personal statements and get some advice. There are lots of websites which will offer you advice for writing a personal statement but it is best to look at websites from reputable organisations such as UCAS or student advice websites which are well used such as The Student Room. Remember to ask your teachers, friends and family for advice. A good question to ask them is "what do you think makes me different from other people? or "how would you describe me?" because the answers often give you some interesting information that you hadn't thought about. The personal statement is not something you can write quickly so be prepared to think a lot about yourself, your qualities, skills, experience, likes and dislikes. You are unlikely to meet the people who read it so your personality needs to come though on paper; this is difficult.