Selecting a month's first day for a date

create table t_ (
  i number,
  d date
);

alter session set nls_date_format = 'dd.mm.yyyy';

insert into t_ values (1, to_date('05.03.2000'));
insert into t_ values (2, to_date('19.04.2001'));
insert into t_ values (3, to_date('25.08.2003'));
insert into t_ values (4, to_date('01.07.2002'));
insert into t_ values (5, to_date('18.02.2000'));
insert into t_ values (6, to_date('21.10.2003'));

select i, trunc(d,'month') from t_;