Catalog Guide 

1@@@@create a new database  using dbca
ORACLE_SID=catalog
ORACLE_HOME=/u01/app/oracle/product/10.2.0/db_1
.......

2@@@@detial 
SQL> conn /as sysdba;
SQL> create tablespace tbscatalog datafile '/u01/app/oracle/oradata/catalog/tbscatalog.dbf' size 200M;
Tablespace created.
SQL> create user ucata identified by oracle default tablespace tbscatalog;
User created.
SQL> grant connect,recovery_catalog_owner to ucata;
Grant succeeded.
SQL> alter user ucata quota unlimited on tbscatalog;
User altered.
SQL> conn ucata/oracle
Connected.
SQL> select count(*) from tab;
  COUNT(*)
----------
         0
[oracle@station78 ~]$ cd $TNS_ADMIN
[oracle@station78 admin]$ cat tnsnames.ora | head -n 12
CATALOG =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = station78.example.com)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = catalog)
    )
  )
[oracle@station78 admin]$ lsnrctl start
[oracle@station78 admin]$ tnsping catalog
...................
Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = station78.example.com)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = catalog)))
OK (0 msec)

@@@Note:
@@@as the client of catalog, my shell assigned the tns is "rmandb"
@@@you must make sure has rmandb point to the catalog db. as the below:
RMANDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = station78.example.com)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = catalog)
    )
  )

3@@@@ example
$rman target / catalog ucata/oracle@rmandb
rman> cretae catalog tablespace tbscatalog;  @@@create catalog ; drop catalog
rman> register database     @@@unregister database;
rman> resync catalog     

$rman target / catalog ucata/oracle@rmandb
rman> set dbid 19293242   @@@if you have many db register the database