The only way to find it is to have activated Find My iPhone on the iPhone, and to have registered it in iCloud's Find My iPhone. Then login to iCloud, click Find My iPhone, and if it is turned on and active it will show up. If it is turned off or someone has found it and reset to factory settings it will not show up.
report the loss to your cell carrier and insurance and see if you can file a claim if it does not show up.
Apple will not be able to help with finding the iPhone.
This happened to my son yesterday when he was out delivering catalogues. As he had Find my iPhone set up (at my insistence after he previously lost a phone at a party), he was able to track the phone's location by using his friend's iPhone. One new iPhone recovered and one very grateful teenager. It pays to ensure that you have a passcode lock set on the phone which will give you more time to find it.