Hi shaniahmad

Before you do anything, first check what are default and temporary tablespaces for the users. NEVER leave them default. Make sure they are not SYSTEM.

select username, default_tablespace, temporary_tablespace from dba_users where username<>'SYSTEM';

If you find SYSTEM tablespace here change them using ALTER USER command. Then you can continue with what rotem_fo has said.

After you transfer all the objects to different tablespaces, you can use the following command:

alter database datafile 'xxxx.dbf' resize 100M;

If this command fails, which is possible, then you will have to export and import the entire database with possibly COMPRESS=Y option.