How to backup a domain
Log into Cpanel
Click the backups icon
Select full or partial backup
Enter your email address for the notification
Wait for email to arrive... logout. You're done.
When the email does arrive, it will look like this...
Ideally, the backup file should be moved off (FTP) of the server and written to a CD. You do this because if the entire site is destroyed, you want to have a backup copy in a place other than on the server.
Your backup frequency is based on the value of your data and how often it changes. If you're not adding copy/data to your site, then a few times per year is fine. If you're storing lots of data on a daily basis, then your backup frequency should be whatever period of time you're comfortable losing.