you first contact your local police station and tell them what happened with you, depending on the severity of the crime, it will be investigated by special cyber crime investigation cell. you can also lodge a complaint through official twitter account of cyber police ..
give specific details of all the facts and the message/material which is offending.
Get a soft copy or snap shot and and print out of the same and enclose the same along with the complaint
while filing complaint -One may need to provide name, mailing address & telephone number along with an application letter addressing the head of a cyber crime investigation cell when filing a complaint.
Superintendent of Police, Cyber Crime Investigation Cell,
Central Bureau of Investigation,
5th Floor, Block No.3,
Lodhi Road, New Delhi 110003