If you are in the United States:
The Department of Health and Human Services has information on each state and how to report.
This site has very helpful information:
The Childhelp USA is a non-profit organizations dedicated to the treatment, prevention and research of child abuse and neglect. The organization provides a broad continuum of programs that directly serve abused children and their families:
If you are interested in information regarding child abuse outside the United States, the United Nations provides current issues and actions that you can research.