How do you use Siri?

Hold down the Home button until Siri engages.

Assuming Siri is turned on in Settings->General->Siri Hold down the home button until the little Microphone icon shows up at the bottom, then ask a question.

Siri requires an internet connection.

How Do I use Siri

http://www.apple.com/ios/siri/siri-faq/

See:

http://support.apple.com/kb/HT4992

and

http://www.apple.com/iphone/features/siri-faq.html

Regards.

Read the information on the pages to which Phil and I linked below. That will give you all the information you need.