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For that, you need to use Find My iPhone application. On the iPhone, go to Settings > iCloud and make sure "Find My iPhone" is enabled.
Make sure you are signed with your Apple ID/iCloud ID there. Also go to Settings > Privacy and make sure Location Services is turned on.
Now you can track it from online (only if it is connected to internet). To track is, go to www.icloud.com from any device.
Click Find My iPhone and this will show the iPhone the map.
If you have any questions or would like any additional information, please let me know.