An Australian Business Number (ABN) is a unique 11 digit number that identifies your business to the government and community. An ABN doesn't replace your tax file number, but is used for various tax and other business purposes.

You will need to register an ABN before anything else, so if you are starting up, think about how you want to structure you business. If you business is going to be a small one person operation then you should consider registering as a sole trader.

If the business is going to be much larger then a registered business or company is the way to go.
A sole trader is a good start up option if you are yet to know how your business will go and it suits your purpose initially, you can always change it as required.

For more information Australian Business Register