Backups are vital to ensure you don't lose all your work.
But there is not much point doing a backup and leaving it on the same computer - what if it gets stolen, the hard drive crashes or whatever...
Using this technique you will download a backup of your website that you can keep on your computer or even better upload to something like dropbox. The more copies the safer. If you are looking for information about WordPress backups, jump here.
Log into your domain management panel: cpanel.123host.com.au
Your login details are in the welcome email you should have received. If you can't find it, lodge a support ticket and it will be resent
Find the JetBAckup section and download the appropriate backup (full, just files, just database).
Keep in mind that 123host backs up all your data every night and keeps various versions of it on an external server for up to 2 months.
How often should you grab a backup? Depends on how often you update your site. If you update daily, I would suggest once a week. If you update once a week, then download once a month. How much of your work are you willing to lose?