Social Work - ONG? Sao Paulo (São Paulo)

Hi guys anyone knows where I can found information regarding collaborating with ONGs or social work organizations here in Sao Paulo. Can be related to children, homeless, drug issues.

There is a lot of information on web, but hard to tell which are serious organizations. I am not thinking of giving money, but actually participating as a volunteer, maybe once a week, or once every two weeks.

Thanks!!!!