To read a message, just click on it. The message will appear in the bottom panel of the Mail and Newsgroups window. 

To make a message open up in its own window, double-click on it, or select the message then press the Enter key.

To go to the next unread message, click the Next button on the toolbar or use the "F" key on the keyboard . To open the previous message in the folder, use the "B" key on the keyboard.

Use the vertical scroll bar (or the Page Down key on the keyboard) to see the bottom of long messages.

You will notice that the email message contains "Header" information to show you the Subject, Date, From, and To details.