To reply to an email press the Reply button, or select Message->Reply to Message alternatively. That will open the message composer. The To: and Subject: fields are already filled, although you can change them if you wish. In addition the full text of the old message is inserted into the new message, with a ">" character in front of each line. This should indicate that its part of the previous message.
If you are reading a message with several recipients you might want to use Message->Reply to All instead.
Message->Reply to Mailinglist tries to determine the list name for you. This is the preferred way of replying when participating in mailing lists.