First contact your mobile service provider to block the SIM of your iPhone. Then check the bill of your iPhone to find out its IMEI number, and contact your nearest police station to report the lost iPhone with its IMEI number.
Use this police report (its is called FIR, or First Information Report in India) to contact your mobile service provider. They will first block this iPhone so that it becomes unusable for the person who found or stole it. It will not work even if they change the SIM. Then they will help you trace your iPhone using its IMEI number.