The screen on the left shows the current threads of conversation, you can read the individual messages just by clicking on the underlined subject heading. If the word more appears under a subject that means that there have already been a number of replies to the first message, clicking on more will expand that section of the contents page so you can see and read the replies so far.
Clicking on the underlined subject of a message will allow you to read the text of the message, the reading screen allow to move to the message before the one that you are reading and to any of the replies to the message you are currently reading. If you want to reply to the current message simply chose the reply option at the top of the reading screen.
If you want to start a new thread, click on the Start New Topic which appears at the top of the contents list.