View menu then (un)click the preview and autopreview options.  

This needs to be done for each individual folder, i.e. go to the inbox and change the option then go to the deleted items and change the option, etc.

FYI: Some menu items do not have a check mark to indicate they are turned on.  Instead, they appear raised or lowered.  In this example, the Folder List option is turned on and the Preview Pane is turned off.

View menu > Layout then uncheck the "Show Preview Pane".  

Note: You must be in one of the folders to turn it on or off ( not available when your are in the Outlook Today folder.)