To take backup in noarchivelog mode you must aware of several things.
1)Don't skip any online tablespace to take backup. If you do exclude an online tablespace to take backup then whenever you restore the backup that tablespace will be lost.
2)You must take a cold backup of your database while taking database backup. Cold backup means your database will be either in mount stage or the instance will be not running.
In next steps I gave an example of how we can take incremental level 0 backup in noarchivelog mode.
1)See the current log mode and Shut down the database.
SQL> SELECT LOG_MODE FROM V$DATABASE;
SQL> SHUTDOWN IMMEDIATE
2)Connect to RMAN.
SQL> !rman target /
3)Mount the database.
RMAN> STARTUP MOUNT
4)Take incremental level 0 backup.
RMAN> BACKUP INCREMENTAL LEVEL 0 DATABASE TAG 'LEVEL_0_BACKUP';
In this example a tag is assigned to level 0 incremental backup.
5)Open the database to use for all.
RMAN> ALTER DATABASE OPEN;