Need to create and RMAN Recovery catalog? Check the Creating a Recovery Catalog section of the Backup and Recovery User Guide document.

Steps listed there are:
1. Create a tablespace for the catalog
2. Create a user that will own the schema and assign to it the newly created tablespace.
3. Grant the RECOVERY_CATALOG_OWNER privilege to the user, and if 11g or higher, grant CONNECT as well.

SQL> create tablespace TOOLS datafile 'C:\ORADATA\RCATDB\TOOLS_01.DBF' size 100M;

Tablespace created.

SQL> create user RCAT identified by RCAT default tablespace TOOLS temporary tablespace TEMP quota unlimited on TOOLS;

User created.

SQL> grant recovery_catalog_owner to RCAT;

Grant succeeded.

SQL> grant connect to RCAT;

Grant succeeded.

4. Using the RMAN utility, login with the CONNECT CATALOG syntax as the recovery catalog schema owner and run CREATE CATALOG. This will build the schema. You can specify a tablespace if desired.

C:\Users\emcadmin> rman

Recovery Manager: Release - Production on Wed Jan 2 18:30:33 2013

Copyright (c) 1982, 2009, Oracle and/or its affiliates.  All rights reserved.

RMAN> connect catalog RCAT/RCAT

connected to recovery catalog database

RMAN> create catalog tablespace tools;

recovery catalog created

At this point one can begin registering within the catalog databases to be backed up.

RMAN> connect target;

RMAN> register database;