You should prepare for an interview as a Peace Corps volunteer like you would prepare for any other interview. They will send you a list of topics that could be covered and you should go over that and practice responding to those questions. In terms of interviewing for a specific sector you should highlight any health care experience that you may have, especially if it was working in a rural, low-income country. Most importantly, you should speak honestly and with passion. Don't talk about experiences just because you think they make you look better, talk about experiences that really meant a lot to you. Speak passionately from the heart because being a Peace Corps volunteer is more about being a kind, passionate, motivated, patient, empathic person more than it is about having certain, field related experiences.
The best way to prepare for an interview position with the Peace Corps is to treat it as a professional job. Familiarize yourself with some of the basic tasks and general topics for Community Health Volunteers, and be prepared to share experience you may have working on related projects (HIV/AIDS, maternal and child health, nutrition, etc.). Dress professionally and bring a copy of your most updated resume, just like you would for any other interview. Bring questions that you have for the interviewer. But also relax, your interviewer is likely excited to meet you and will have great Peace Corps stories of their own to share!
Update resume and references to be completely up to date. Be prepared to answer questions about times when you were in charge and had to make decisions or plan events. And be sure you have an idea of what kind of work you want to do or where you want to go. If you are completely open to the work and location of your service, that is fine too, just make sure to talk about why you want to go into the Peace Corps and your passions.