To find a stolen iPhone, sign into iCloud.com/Find on a computer, or use the Find My iPhone app on another iOS device. To find a stolen iPhone, Find My iPhone must be enabled in your iPhone's settings.
Next, choose your device to see its location on a map. Once you are near the device, use the app to direct it to make a noise so that it is easier to find.
If you did not turn on Find My iPhone before it was lost or stolen, the feature does not work. Find My iPhone is the only way an iPhone owner is able to track or locate a missing device short of searching manually.
However, there are several ways to prevent a thief from accessing your data. Change your Apple ID password so that no one is able to access your iCloud, iMessage or iTunes data or services. It is a good idea to change passwords for Internet accounts and email accounts as well.
Finally, report a stolen iPhone to local law enforcement officials. If you kept the original packaging or a sales receipt, give the device's serial number to the police, who might be able to match it to your phone if it is recovered.