First, if your school doesn't contact you about an interview, don't worry! Even those schools that strongly encourage interviews don't necessarily reach out to every candidate with a chance of acceptance. Moreover, even at schools that encourage applicants to interview, the interview is nowhere near the most important part of your application packet. Nevertheless, you'll want to put your best foot forward in order to give yourself the best chance possible, so here's how:
Most importantly, interviews are often your best opportunity to illustrate both your actual, lived personality AND your professionalism. In general, try not to develop one at the expense of the other. Even at the most formal of schools, interviewers will want to hear genuine excitement in your voice as you talk about your achievements and experiences. Just make sure that you practice with at least one or two adults so that you can be positive that your excitement comes across as genuine and not sarcastic or otherwise fake.