The general rule is you need an ABN number if it is a "Business" if it is a "Hobby" then you may not need an ABN number, but that is a grey area.  I got an ABN number when I was 18 to sell on eBay the right way and pay tax. Now a few years later ive moved to a company and pay GST. My advice is if the businesses not a hobby set up and ABN number it just looks more processional when you send invoices to clients ect.