Hello,
I am assuming that you are in year 12/first year of A Levels or the equivalent. In this case, you should demonstrate your interest in the subject by going above and beyond your studies and making use of various extra-curricular activities. One really good thing you could do which I would highly recommend is an Extended Project Qualification where you could look at a different area in Psychology. I did an EPQ in Leadership Psychology and not only was it highly interesting but the skills developed from an EPQ are really helpful for university education!
I hope this helps, if you have any further questions please do let me know!
Charlie
Law LLB Student