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>