Apple offer a free service called Find my iPhone which lets you track your phone if you've misplaced it. Find my iPhone is compatible with:
You can make your phone ring, make a message appear on the screen, set up a passcode and even erase all data on your phone if you don't think you'll get it back.
You'll need to make sure that your iPhone is signed in using your Apple ID and that Find my iPhone is switched on.
To make sure Find my iPhone is switched on: