Siri comes to life with the press of a button.

Start Siri:  Press the Home button   until Siri appears.

You’ll hear two quick beeps and see “What can I help you with?” on the screen.

Just begin speaking. The microphone icon lights up to let you know that Siri hears you talking. Once you’ve started a dialogue with Siri, tap the microphone icon to talk to it again.

Siri waits for you to stop talking, but you can also tap the microphone icon to tell Siri you’re done. This is useful when there’s a lot of background noise. It can also speed up your conversation with Siri, since Siri won’t have to wait for your pause.

When you fnish speaking, Siri displays what it heard and provides a response. Siri often includes additional, related info that might be useful. If the info is related to an app—for example, a text message you’ve composed, or a location you asked for—just tap the display to open the app for details and further action.
Siri may ask you if it needs more information to complete a request. For example, tell Siri to “Remind me to call mom,” and Siri may ask “What time would you like me to remind you?” Don’t worry about telling Siri everything at once. Siri prompts you for the details it needs.

You can start talking to Siri just by bringing iPhone to your ear, like making a phone call. If the screen isn’t on—frst press the On/Of or Home button. You’ll hear two quick beeps to indicate Siri is listening. Then start talking

If Siri doesn’t respond when you bring iPhone to your ear, start with the screen facing you, so your hand rotates on the way up.
Cancel a request : Say “cancel,” tap "miscro icon" , or press the Home button 
Stop a phone call you started with Siri : Before Phone opens, press the Home button  . If Phone is already open, tap End.
Suitable headset for iphone 4s :