If you want to change the way the reading pane for your emails is displayed (or not displayed), you can change this under the Reading Pane Settings.
Outlook on a Windows computer
Click on the View tab > Reading Pane > and then select how you want to view your reading pane.
Outlook on a Mac computer
Click on the Organize tab > Reading Pane > and then select how you want to view your reading pane.
Office 365 email
You can change the view in Office 365 by clicking on the gear icon and choose Display Settings > Reading pane > and then selecting which layout they want. You then should press the ✔OK icon.