Launch the Messages application on the iPhone by tapping its icon on the home screen. A list of previous text message conversations will be displayed, with the newest additions at the top of the screen.
Tap the entry for the conversation you want to read. A history of messages sent between you and the other party will be displayed in chronological order.
Scroll to the bottom of the conversation window to read the latest text message received.
Tap the text field at the bottom of the screen to type a reply to the current conversation, and then press the "Send" button. Press the "Messages" button in the upper-left corner to return to the list of existing conversations.