Explain plan for an SQL query

1. Create the plan table

SQLPLUS> @${ORACLE_HOME}/rdbms/admin/utlxplan.sql

2. Delete old plan from the table and run explain plan. 
Replace 'select * from v$database' with the actuall sql statement you want explain plan for.

set lin 180
set pages 0
set feedback on
set echo off

delete from plan_table
where STATEMENT_ID = 'p1'
/

EXPLAIN PLAN SET STATEMENT_ID = 'p1' FOR 
select * from v$database
/

SELECT LPAD(' ',2*(LEVEL-1))||operation||' '||options
        ||' '||object_name
        ||' '||DECODE(id, 0, 'Cost = '||position) "Query Plan"
FROM plan_table
        START WITH id = 0 AND statement_id = 'p1'
        CONNECT BY PRIOR id = parent_id AND statement_id ='p1'
/