Open a thread and use the action bar at the top:
- Reply — replies to the sender; the quote of the original message is added below your reply automatically.
- Reply all — when the message had multiple recipients.
- Forward — opens a field for the new recipient address, with the full message body as a quote.
Your reply is sent from the same account where the message came in, so the other party stays in the same conversation thread. After sending, the thread refreshes immediately with your new message at the bottom.