Creating RMAN database to manage recovery catalog.

RMAN DBname: RMN1
tablespace name: rman_tbs
datafile name: rman01.dbf (size 100M)

on the prompt:
$ dbca &

step-1:
pick singel instance database on the Database Configuration Assistant: Welcome Screen.
step-2:
Pick Create a Database option
step-3:
Pick Custom Database on the database template. This is because you don't want any 
sample schemas in you RMAN database.
step-4:
Type "RMN1" rman database global name
Step-5:
Disselect "Configure the Database with Enterprise Manager" option, this is due to the 
fact that you're creating rman database to manage your recovery catalog and you don't
want to overload this small database with stuff unneccesary.
step-6:
Type password for sys/system login and save it in the safe place as we will require this
login credentials later to manage catalog database.
step-7:
Select "File System" option from Storage Mechanishm
step-8:
select "Use Database File Locations from template" option to choose for datafile 
storage or you can choose Oracle-Managed Files option.
step-9:
Make sure you Specify Flash Recovery Area
Step-10:
Choose appropriate memory for the rman database depending upon your need.
i go with about 300MB SGA size and 120MB for PGA.
Step-11:
Confirm the summary of parameters and other items and click Finish to start database creation.
Verify database creation:
$ export ORACLE_SID=RMN1
$ sqlplus rman/password@RMN1
Now you are ready to create rman recovery catalog database and register target database.
Go to the following link:

Creating a Rman Recovery Catalog