Badri,

It is very simple. Hot backup's are done when the database is online. What you have to do is backup every tablespace one after another. You cannot backup the whole database in one step. Beofre you backup issue the statement:

ALTER TABLESPACE tablespace_name BEGIN BACKUP;

Now back up all the datafiles that belong to that tablespace.
End the Backup :

ALTER TABLESPACE tablespace_name END BACKUP;

Hope this helps.

Nizar