Try to make yourself sound well rounded academically and extra-curricularly (don't think thats a word but oh well), and like you would be a member of the college that would be an active participant and give back to the college community. They almost all have more applicants than places, so why would you contribute/add more to the college than someone else? You could also try to talk about how you would balance the distractions of college life with academic work, and your suitability for living with others, and I would also try to talk about why you chose that college specifically, to show them you are interested in that college and not just applying everywhere and hoping to get in somewhere.
I know that is a lot for 300 words but its some ideas I guess.
During the holidays I had an interview at Sancta, not sure if you are even female or applying there but you are welcome to private message me and I can tell you the sort of things they asked me in case thats any help