How to use simple output parameters?
If you are still using SQL*Plus variables

variable tmpString VARCHAR2(100);
begin
  myTestProc( :tmpString );
end;
/
print tmpString;


If you want to do it in PL/SQL

SET SERVEROUTPUT ON;
DECLARE
  tmpString VARCHAR2(100);
BEGIN
  myTestProc( tmpString );
  dbms_output.put_line( tmpString );
END;