My thought, having taught English in the USA, for 30 years, is that you have broad interests (good) and strong motivation (also good), but that you need to focus on interview skills AND what's going on the the U.S. As the exchange program is going to pick students who show a strong interest in The States, you need to bone up on the important news here. One way in would be to read at least the major articles that begin on the front page of the New York Times (on-line addition) every day. The nytimes.com edition of the paper has a free subscription. You simply fill in a few blanks, and it will start appearing in your e-mail inbox. Another good source for news of the U.S. is the BBC on-line news page, in the section marked "Americas." Currently, you might want to give most attention to stories having to do with the economy, education, health care, and the upcoming election of a new president. I suggest the New York Times because it is the most serious (and, usually, the most accurate) of easily-available American newspapers.
As for the interview skills, they are just that: skills. You can master them quite easily with a bit of practice. The program you want sounds academically oriented; therefore, if there is anyone you know who has recently taken (or, ideally, given) a university admissions interview, ask if he or she would be willing to run a pretend version for you. If you could do this with even two or three people, that would be a big help. Ask them to slant their questions towards some important information about the States. Even if you don't know such a person, one or more of your high-school teachers would likely be able to give you a make-believe interview. A History teacher? An English teacher. A teacher who has traveled or taught or been in school in the States?
For 20 years, I was in charge (in the northern half of Maine) of the interviews that high school students sat for with alumni/ae for admission to Yale College. I can assure you that, for selective colleges and programs (and the program you seek sounds highly selective), the interviewers are asked to make the interviewees as comfortable as possible, not bombarding them with them ridiculously difficult questions, but, instead, giving them an opportunity to demonstrate what THEY know. You are very likely to be treated extremely politely. Do have ready a couple of significant questions about the U.S. to ask the interviewer; that will confirm in his/her mind your strong interest.
Hope this information is useful to you. Best of luck!
P.S. Our younger daughter spent an exchange year in a high school outside of Brisbane. She loves Australia and looks forward to going back for a long visit.