Follow the following step to create new user/schema in oracle 10g.
1 ) Login with existing user
2) Fire following command ..
CREATE USER <new User Name> IDENTIFIED BY <new User Psw>;
new user will created
3) Give session permission to user by issuing following command
GRANT CREATE SESSION TO <new User Name>WITH ADMIN OPTION
4) Now exit from old user account
5) Login with new username/psw@database name in command prompt
EX: sqlplus abc/abc@mydb;