explan plan for <your query>
for example...
explain plan for
select * from dual;
then to see that plan...
select * from table(dbms_xplan.display)
To trace your session in 10g and higher use dbms_monitor.
in 9i, you
alter session set timed_statistics=TRUE;
alter session set sql_trace=TRUE;
even better is (officially unsupported, but well documented by Oracle support as an approved method)
alter session set events '10046 trace name context forever,level 12';
or, if you have the dbms_support package installed, use it.
