an ABN means you also need to register at the department of fair trading if your business name is something other than your own name. it does cost money and i still haven't gotten around to doing mine, so really, someone can just come up, take my business name and sue me for using it.
as far as tax is concerned, you keep your own invoices that you issue to clients. you also can claim business expenses - anything you buy work related (the term work related is quite broad) and as long as it's either unaddressed or addressed to your business name/address, you can write it off as a business expense when doing your tax. if it's an invoice, it's also important to show evidence that you've paid for it, so if you pay them via netbanking or bpay, print the receipt and staple it together. you can document these costs and revenues into an excel sheet when you receive them or do them at the end of each financial year (you have up until november to submit your tax). to do your tax, you can just use the etax program from the ATO website.
also note that if you currently get a youth allowance or something from centrelink, you'll need to inform centrelink that you run a business. which means you will also need to generate profit & loss statements, balance sheets and what not. i'm doing this at the moment and i report in fortnightly my earnings.
profit and loss statement is exactly that, you list your earnings, list your expenses and then summarise it with some simple maths.
balance sheet is an outline of EVERYTHING that happens in your business - your assets worth (equipment, property, etc), debts, income, expenses, etc - all using numbers. and then after all the adding and subtracting, you will end up with a balance.
yes owning an ABN is a fucking pain in the arse and you may choose not to have your p&l statements and what not. but when you're doing something that involves the evaluation of your business and its worth, you better have them ready or you'll be on the shitter with your pants on.
