Step 1: Set the following environment variables

ORACLE_SID
ORACLE_HOME
NLS_DATE_FORMAT

Step 2: Access to privileged OS account or user with Sysdba privilege. Only users with Sysdba privilege can connect to RMAN. Hence no need to specify as sysdba while connecting to RMAN.

$ cd $ORACLE_HOME/bin
$ rman target /

Recovery Manager: Release 10.2.0.1.0 - Production on Wed Jan 20 03:19:22 2010
Copyright (c) 1982, 2005, Oracle. All rights reserved.
connected to target database: IRIS (DBID=1460454352)

If you are using password file, then specify the username and password

$ rman target sys/syspassword

If you are accessing the target database via oracle net then use connection string as follows

$ rman target sys/syspassword@

You can also connect to RMAN in two steps first by invoking the rman

$ rman

The RMAN prompt appears

RMAN>

To connect to the target datbase..

RMAN> Connect target /
RMAN> Connect target sys/syspassword
RMAN> Connect target sys/syspassword@

Once you have connected to Rman, you can start entering the commands. The commands can be typed in single line or in multiple lines and ends with a semicolon.

RMAN> Backup
2 database;

To add comments use the # symbol. Text after the # symbol in a line is interpreted as comments

RMAN> # Backup database
Backup database # this command will backup the database
include current controlfile; # backup control file along with database

Thanks