How to Find a Lost iPhone

Losing any phone is a pain, but losing your iPhone means a significant cash outlay for a new one, plus the loss—and possible exposure of—personal data, your iTunes account, your photos, and all the hours you've already spent setting it up just the way you like. Perhaps, all is not lost! If your iPhone hasn't been stolen—you have merely misplaced it—your chances of recovery are excellent, and we'll show you how to do it.
Steps:

1.   Open the "Find My iPhone" app on an iOS device or iCloud on a Mac. If you don't have the app you can get it from the app store.You will be asked to log in again, as a security precaution. Do so, and after a moment, you will be taken to a screen. It may default to a map of Apple in Cupertino, or it may display a map showing you the location of your phone.
2.  Click on the Devices button. This is in the upper left of the screen, and will give you the option of choosing any enabled device.
   
   
   
3. Your iPhone will be displayed in this list. Click on it, and Find My iPhone will search for your phone's location.
4. Note: If the phone has been powered off, or the battery has run out, it will show you the last known location, but will not be able to show you the current address.
5. Send a notification. From your iPhone's info window, you can send an alert. If your phone has slipped between the cushions, or left in your jacket, this makes it easy to find. For two minutes, a loud alert will sound—even if you've muted your phone.
   
   
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7. Track your phone. If you're using iOS 6 or later, click Lost Mode in your phone's info window.
   
   
   
8. You'll be asked to enter an unlock code for your phone. Use a random number set that's not tied to you: no SSN, no birthdate, driver's license number, or anything that is personal.
9. You may also enter a message and contact phone number.
10. If your iPhone is online, it will immediately be locked, and you can see your phone's current location, as well as any changes in location. If your phone is offline, it will lock immediately if it's powered up, you'll be notified via email, and you'll be able to track your phone's position.
11. Erase all your data. Should you not be able to locate your phone, and suspect that it's lost, click on the Erase iPhone button in the phone's info window. This will erase all data, but you will not be able to use Find My iPhone to locate your phone once that happens. It's better that, however, than to have your personal information compromised.
    
    
    
    Should you later recover your phone, you can restore your data through your iTunes backup.

Warnings:
When you wipe your iPhone it will remove all of your data: your contacts, mail, applications and everything else.
Things You'll Need: 

• A iCloud account
• An iPod Touch, iPad, or another iPhone
• An Apple ID
• A computer

(Source: www.wikihow.com)