Now you might be thinking how does it works ? well when you will enable the Activation Lock in your device you will need an Apple ID which is securely stored on Apple's activation servers and directly linked with your device. From that point on, your password will be required before anyone can turn off Find My Phone on your device, erase your device or even reactivate and use your device.
Your activation lock will be enabled automatically when you turn on Find My iPhone in iOS 7. If you updated to iOS 7 recently don't worry in your previous version of iOS that has Find My iPhone already turned on.
To activate your your Activation Lock on iPhone or iPad or iPod Touch you will need an Apple ID and password. If you don't have Apple ID yet create one now - How to open Apple ID
How to enable Activation Lock Find My iPhone
Step 1: Go to Settings.
Step 2: Tap on iCloud.
Step 3: Sign in with your Apple ID and Password (if required).
Step 4: Turn on Find My iPhone.
Activation Lock works only with iPhone, iPad, or iPod touch devices running iOS 7 it will not work with Mac device.