Welcome to Android Central! Can you be more specific? What device are you using, and what version of Android is it running? Are you trying to dictate a text message in the Messaging app, or Hangouts? If you bring up the keyboard, there should be a microphone icon there--tap it, and start talking when prompted. It should transcribe what you say.
Or are you talking about having the phone read a text message to you out loud, using text-to-speech?