I started off with mentioning certain topics that I have studied during high school and emphasised that I enjoyed them due to their close relation with economics. ex. certain topics covered in A level Maths.
You want to show that you are going out of your way to understand more about the subject you are applying for ex. reading books (In first year at St Andrews we cover The Economy by The Core), Participating in work experience or free online programmes, reading articles online on The Economist.
You want explain what you have learnt and show that you have understood what you have learnt about. you can pick out a small topic that you have enjoyed reading about, potentially state your opinion/insights and relate it to an event you may have read about in the news. I remember I mentioned inflation as that was a big issue at the time. I think the field of Behavioural Economics is easy to explain and show your interest in however it is also quite commonly used in personal statements.