How to activate, setup or configure Siri on the iPhone 5
All you need to do is to go to Settings > General > Siri. Repeat the process to turn Siri on or off.
Now that you have Siri setup, how to ask or use Siri.
Hold down the home button till you hear two quick beeps. A microphone icon will light up and you should see the question 'What can I help you with?' on your home screen. Tap on the microphone icon to begin speaking. If you have your headphones on you would need to hold the center button to talk to Siri. To use Siri with Bluetooth hold the call button to bring up Siri.
How to troubleshoot if Siri is not found under the settings option.
Try setting your region format to US. This is if you're in another part of the world. Siri will get turned on and you can begin using.