As Jeruba said, you’d want to start with your local hospitals. However, due to security concerns that are taken with babies, I doubt that too many will allow volunteers to work with the babies.
When I worked in a children’s hospital, there were some babies that went to a newborn orphanage while awaiting foster care placements or for their mothers to finish rehab. Perhaps you could investigate whether there is such a facility near you, because they would probably love volunteers.
Keep in mind that if you have yet to turn 18, you might not be eligible for any such volunteer opportunities depending on regulations where you live. You may want to try to find babysitting jobs or maybe be a mother’s helper for a woman with multiples.