Q: Siri

You must be running iOS 6 in order to have Siri. If you are running iOS 6 - go to Settings>Siri>On. Once enabled, hold down on the home button until Siri pops up on the screen.

If you are on iOS 5.1.1 as your tagline suggests then you first need to update to iOS 6 . You can then enable it via Settings > General > Siri, and you can then use it by pressing and holding the home button (you will need to be online to use it)

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