Activate Find My iPhone/IPad

 Make sure you have the latest iOS 5 update installed and i Tunes 10.5 or later.

1. Tap the Settings app on your iPhone. Then Tap "Mail, Contacts, Calendars" and "Add Account." Then choose MobileMe. 

2. In the MobileMe account menu, enter your iTunes or Apple ID and password (i.e., the login you use to buy iTunes media on the iPhone).

3. The "Find My iPhone/IPad" option should appear. Slide it to "ON" to activate it."

To use find My IPhone, go on a computer and enter  www.icloud.com in a web browser. Then enter the same login credentials you used to register for Find My iPhone, and you'll immediately get a GPS reading of the phone, along with a simple menu of buttons allowing you to :
lock, wipe, or send a message or sound to the iPhone.