create database failed and database already mounted ?

Question

when i create a new database in sql plus it shows the following error

here even i applied startup nomounted again it shows same error.. pls help me to how to create the new database? and how to solve this problem?
SQL> conn system

Enter password: *****

Connected.

SQL> create database new_db;

create database new_db

*

ERROR at line 1:

ORA-01501: CREATE DATABASE failed

ORA-01100: database already mounted

database name is orcl and connect as sysdba