A good personal statement should start by being clear and outlining exactly why you want to study the course. This is an opportunity to show that you have researched the course and therefore can comment on particular modules you look forward to studying. You can also demonstrate how your previous studies has developed your enthusiasm for the course.
It is important that a personal statement includes reasons why you are suited to the course. Here you can state that you fit the necessary criteria for the course but also that your personal skills are suited to studying at university level.
It is a good idea to demonstrate any extra-curricular activities or opportunities you have taken up which demonstrate your interest in the course. This could be extra reading, volunteering experiences or work experience. It is always important here to remember to explain how these are relevant to the course you are applying for, including what you have learnt from them and how they will help towards your chosen career. This may only apply for certain vocational courses which ultimately lead to one profession (i.e. medicine or law).
It is important to discuss the skills you already possess and how these will help you when studying at degree level. You can explain how you have obtained these skills, for example through projects or assessments, extra-curricular activities or groups, volunteering or a part time job. An extra tip here would be to demonstrate that you are a critical thinker as this is something you would need to be able to degree at university.
You should end by discussing any long term plans you have for example further study. This is an opportunity to show your enthusiasm for pursuing a career by completing the chosen course.
JUST REMEMBER, universities read so many personal statements, the key is to stand out. When I say stand out, I don't however mean use lots of clichés, quotes from famous people or big statements about yourself with no evidence to back them up. Be genuine, give examples of why you have a particular skill and stay clear of overused phrases like 'from a young age' and 'I have a passion for'.