Start with 'I decided I wanted to study xxxx at University WHEN........' and think/write about what it was, something you read, a topic in class, a documentary etc etc. What was it that made you decide this - and why.
Then write two chunky paragraphs that show your academic interest in the subject, not about A level course work topics, but stuff beyond that/higher level than that. 'I read .... and that motivated me to find out more about ......', 'This interested me because.....' etc etc.
Short final paragraph - interesting other stuff you do that you can link to personal qualities (not just a list of hobbies), and 'I look forward to all the opportunities and challenges that being at University will bring' or similar. Job done.