Audit Database Login and Loogout

I work on oracle 9.2.0.4

I created a table called t_audit (oper_type varchar2(50), username archar2(50), machine_id varchar2(50), oper_date date)

to store in it all the operations worked on the database like  login, logout, open db, shutdown

how can I create a trigger on the database level to fill the table t_audit  with the required data.

Thanks in advance