It is not required to have a separate repository for RMAN because, by default, RMAN will write records to the database control file.
To take a complete database backup, start RMAN from the OS command prompt:
> rman target /
Then issue the RMAN backup command:
RMAN> backup database plus archivelog;
The location of the backup set (files) is dependent on release and RMAN environment settings. In 10g, by default, RMAN writes the backupset to the flash recovery area. You can check that location from sql*plus:
SQL> show parameter db_recovery
I have a series of free online tutorials here –
Hope this helps.