as long as you have siri (ios5/6) you should be able to use the microphone button on your keyboard to begin entering text via voice... ?
The above is correct. The OP asked the question a bit vaguely and the Mod even seemed to be a bit confused.
TO The OP, sending a text via voice is simply done with your iPhone 4S by pressing the home button until Siri pops up. If it doesn't, Siri must not be enabled for some reason. Go to settings > general, and Siri should able an option. Press it and you'll be able to turn Siri on if it wasn't on already.
As what to say to Siri, simply tell it to create a text message to send to whatever contact you have or phone number. You should be able to dictate the message and Siri will confirm whether you wish to send the message.