There are many organisations dedicated to advocacy for preventing genocide and mass atrocities. Some useful links are below. Explore the websites of these organisations - most have a section to 'Get Involved'.
Auschwitz Institute for Peace and Reconciliation:
http://www.auschwitzinstitute.org/
Genocide Watch:
http://genocidewatch.net/
The Sentinel Project:
https://thesentinelproject.org/
Enough: The Project to End Genocide and Crimes Against Humanity:
http://www.enoughproject.org/
Aegis Trust:
http://www.aegistrust.org/