You can use to drop database DBCA if u create database using DBACA.
Otherwise you have following one of following method
****************Method One**********************************
sqlplus / as sysdba
shutdown immediate;
startup mount exclusive;
Alter system enable restricted session;
drop database ;
****************Method Two**********************************
sqlplus / as sysdba
shutdown immediate;
STARTUP NOMOUNT RESTICT;
ALTER DATABASE MOUNT;
DROP DATABASE;