1. Create or replace trigger trigger_name.

2. if u are using many DDL's like this make sure u have a '/' to mark the end;

CREATE or replace TRIGGER test_trigger
  BEFORE INSERT ON test
  FOR EACH ROW
  BEGIN
  SELECT test_seq.nextval INTO :new.id from dual;
  END;
  /
  CREATE or replace TRIGGER or replace test_trigger
  BEFORE INSERT ON test
  FOR EACH ROW
  BEGIN
  SELECT test_seq.nextval INTO :new.id from dual;
  END;
  /