As a pre nursing student at CSULA, I started volunteering when I was in high school, and luckily I was able to submit those hours as they met the cutoff. I believe there were instructions that stated you could only submit hours that date back to 1-2 years ago. If you start accumulating hours now and apply for the program in the fall they should count. As for any previous interviews before volunteering, I remember having to do a phone call interview when I volunteered at Kaiser, after I submitted my application. However, this was just my experience at one hospital, so I'm sure you'll have to look into the requirements and application process at your respective hospital.
Most people I know got their volunteer hours at local hospital, but I think volunteering at clinic or something similar should be fine, as long as it involves some sort of patient care (giving water, blankets or feeding, etc) and a person to write a letter confirming your hours and services.