If you have the iPhone 4 - Siri is not available. You need the iPhone 4S or the iPhone 5 in order to have Siri.
*Siri is available in Beta only on iPhone 4S, iPhone 5, iPad with Retina display, iPad mini, and iPod touch (5th generation) and requires Internet access. Siri may not be available in all languages or in all areas, and features may vary by area. Cellular data charges may apply.