Update: iOS 9 disables the "Hey Siri" feature, and forces you to follow onscreen prompts to enable it on your device. You can learn how to set up Hey Siri in iOS 9 here.
Apple added the ability to wake Siri with your voice by saying the phrase "Hey, Siri," but you need three things happen before you can make the new feature work:
1. Your iPod touch, iPhone or iPad has to support and have iOS 8 installed.
2. Your device has to be connected to a power source / charger.
3. You have to turn on the Voice Activation feature by navigating to Settings > General > Siri.
Once the Voice Activation slider is ON and your device is plugged in, just say "Hey, Siri..." followed by your question and Apple's voice assistant will help you.