Creating Oracle Sequence Trigger

Try this:

CREATE OR REPLACE TRIGGER TRIGGER1
BEFORE INSERT ON ACCOUNTS
FOR EACH ROW
  WHEN (new.ID IS NULL)
BEGIN
  :new.ID := ACCOUNTS_SEQ.NEXTVAL;
END;
/

Your sequence should look like this:

CREATE SEQUENCE ACCOUNTS_SEQ 
  START WITH 1 
  INCREMENT BY 1;

CREATE OR REPLACE TRIGGER TRIGGER1
BEFORE INSERT ON ACCOUNTS
FOR EACH ROW
BEGIN
IF :new.ID IS NULL THEN 
SELECT ACCOUNTS_SEQ.NEXTVAL 
INTO :new.ID
FROM dual;
END IF;
END;