Not sure about over there, but here, our government has a website we can go to which explains it all... And as somone mentioned, they will most likely have a program that you can download which has all the fields to fill in, and that will estimate your return, and lodge all the documents for you..

My girlfriend did her's via one of these programs last year, but accidently ticked an incorrect box, and ended up getting a tax bill for $1700.... It took a long time as well to get it cancelled and a new one lodged,, which she had to go to a professional for them to fix it..
It ended up that she got $450 back instaed of having to pay $1700,,, but it took weeks to fix up and had to pay the accountant more because they had to fix up the problem as well as lodge a new return...

So if you do your own, just be careful that you check and re-check everything twice before lodging it.