Q: how to get lost iphone

If you activated the Find My iPhone service (in Settings>icloud), and if the device's batteries still have a charge, the device is not turned off and has not been reset and is connected to wifi, then you can try to track it.  Using a computer, log into icloud.com and look at the Find My iPhone page.  On a device, invoke the Find My iPhone app (get it for free at the itunes app store) and log into the icloud account.

If the device is listed as "offline", then one of the requirements listed above is not met (like being connected to wifi, or if the device was stolen and the thief reset it).

If the service is not turned on, then there's nothing you can do.