Being a CNA at a hospital is hard work (as is any CNA work) and usually (in many hospitals) you would work 36 hours weekly, in 12 hour shifts, 3 days per week. As a hospital CNA on oncology, I bathe patients, toilet patients, empty foley and ostomy bags, take vitals, order meals, feed patients who can't feed themselves, take various samples for the lab (sputum, stool, urine), walk patients who need walks, sit with patients who need sitters, assist nurses with dressing changes, and take blood sugars (many, many blood sugars).
Hospitals can be very dangerous places as most have an open door policy, we have had many incidents that have put staff and patient safety at risk. For this reason it seems many hospitals might not take on the liability of hiring an underage CNA. A nursing home might hire you or an assisted living facility. Just stick it out wherever you can get hired and then do hospital work when you turn 18.
Good luck! Being a CNA is wonderfully rewarding work.