You have to create a recovery catalog first.
Create a new database or pickup an existing database.
Create a tablespace for RMAN objects, about 50 MB size.
Create a user 'rman' and set his default tablespace as that created above.
Grant RECOVERY_CATALOG_OWNER to rman.
Grant DBA, Connect & Resource roles to rman.
Now connect to recovery catalog as user rman to create recovery catalog objects.
As much as possible create your rman in another database or much better if in another machine. Otherwise, your rman will defeat its intention if something goes wrong with the database that you intend to apply with rman.