What is it that really interests you about the subject? Have you already identified some particular areas of interest?
Have you already studied your chosen subject at A-Level (or equivalent), or have you covered areas of it whilst studying other subjects?
Have you done any wider reading on the subject?
Do your career aspirations relate to the subject you want to study?
Have you completed any relevant work experience?
Do your extra-curricular activities relate to your course?
If you are taking a gap year, are you doing anything that will help you to prepare for your course?