Where are you in California? Bay Area? Not too many volunteer experience three. You could try
Rock Medicine. To join, you go to an orientation that they have about once a year usually mid April either in Mt. View or Marysville.
You could get some easy certifications out of the way too such as FEMA classes. Usually ICS 100, 200, and NIMS 700 are the bare minimum for most ambulance companies.
Also I am assuming you are gonna try to get an EMT ambulance job. Straight up, just apply everywhere. Don't be picky about whether it's 911 or not.
For interviews, I've had a couple of bad ones personally. I have not successfully gotten the job I wanted. First, I know customer service is huge! They suck that stuff up. Then also evaluate your life. Why do you want to be an EMT/paramedic? Why do you want to be in Fire? Where do you see yourself in the future? What are things you have done to help the community? Think about your education, what you have done with your time off, and what you have to offer. It's best not to prepare for interviews by trying to determine what kind of questions they are gonna be ask, but by thinking about things in categories prior to your interview. Look up generic interview questions eg "Define customer service", which I've been asked in three interviews.
For your resume, I hear firefighters love resume paper. It's the kind of paper that you can hold up to light and see some sort of text through it. Meh.
For your interviews, bring color copies on individual paper of everything you have.
Research who you are apply for. Sometimes they'll specifically tell you what to do eg what to wear to an interview, what to bring, what are the qualifications, etc.