Your introduction and even your first sentence are the most important part of your personal statement. This is where you can grab the attention of the admissions representative. Make your first sentence unique and compelling. It is recommended that you state in a creative way WHY you want to undertake this field of study in your first sentence. The rest of your introduction should provide a brief explanation that supports this first statement. Divide the word limit by the three parts of your personal statement by allocating your introduction around 30%.
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.