If it is something like BSc/MBBS that you are looking at doing then St. Johns is a good option. I don't go to that college personally but I have quite a few friends who do. From as far as I can tell St. Johns and Emmanuel have the largest cohort of premed students which is good for support when it comes to uni work. I can't speak to the interview but the college on a whole is one that recognises and rewards excellence. Colleges want to know that you will be an asset to them, not just someone living in their halls and St. Johns is fairly competitive so it would be good to show that you can somehow help them win. Success and a willingness to participate and compete in the following areas would be great to convey in a college interview:
Sporting: Tennis, netball, soccer, AFL, swimming, athletics, cross country..........(most sports really)
Cultural: Debating, Dance (there is a college 'dancefest'), Singing and other music (Choralfest and Bandfest), and public speaking (inter-college oratory)
Academic: St. Johns rewards high achievers and takes not of your OP for admission. Showing them that you are academic now, and plan to succeed at uni would be good.
Spiritual: This one can be a bit more tricky. I think John's is anglican (check!) and letting them know you are a member of that church would be excellent. But take heed don't try to fake it, I have a friend who is a non practising anglican and he was caught out in the interview because he was unable to name his bishop/congregation or something.
The usual interview stuff like teamwork, motivation and all that jazz also applies. Good luck