Hi. I have been an EMT and EMS instructor in CT for many years. Each state is different in its EMS regulations, but many recognize the national standard for EMT training (NREMT). Contact your state's Department/Office of Emergency Medical Services, mostly likely a division of the Department of Health. They can give you info on teh type of certifications available to you in your area.
In CT we have 4 levels of certification:
MRT - medical response technician (first responder) ~40 hour class
EMT- Basic ~120 hour class
EMT- Intermediate ~80-100 hour class plus EMT class
EMT- Paramedic ~1-2 year program
A basic ambulance service in my region rides with a minimum 2 EMTs or an EMT and MRT. An ALS unit can have an EMT-I and EMT-B or EMT-P (if it is a paramedic unit). Volunteer services often ride with 3 techs, paid services stick to the minimum staffing. Each service has its own rules regarding observers. At the very least, all observers must have CPR, Haz Mat and Blood Bourne Pathogen training.
Volunteering for an ambulance or rescue squad can be a good way to go. Many volunteer organizations will reimburse you for your tuition if you volunteer for them for a minimum time (6 months to 1 year usually).
Call around to some ambulance services in your area and find out what type of opportunities are available in your area.
It is a great experience and there are some really amazing people in EMS.