Each complete line of the PL/SQL code must end with a semicolon (;). : Introduction « PL SQL Programming « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. PL SQL Programming
3. Introduction

To run the code, type / at the beginning of the first blank line after the last line of the code.

SQL> set SERVEROUTPUT ON
SQL> declare
  2      v_string varchar2(256):='Hello, World!';
  3  begin
  4      dbms_output.put_line(v_string);
  5  end;
  6  /
Hello, World!

PL/SQL procedure successfully completed.

SQL>