PL/SQL serveroutput ON

Whenever you start Oracle SQL (PL/SQL) at that time you must have to write the "SET Serveroutput ON" command.
We always required to get serveroutput result and display into the screen. In order for you to see the result , you must tell SQL*Plus to display it. This is done with the SQL*Plus command : 

                     SQL> set serveroutput on 

Note: It needs to be executed only once per session, so you won't need to reissue it unless you exit SQL*Plus and get back in again. It is SQL*Plus that prints the server output on the screen for you to see. You must remember to execute the SET SERVEROUTPUT ON command, or you won't see any output. You also can use the SET SERVEROUTPUT OFF command to turn off output when you don't want to see it.

Set serveroutput on Example