You will need an ABN....
And depending on how much you earn, you may have to charge GST and lodge monthly or Quarterly BAS statements.
You will need to pay income tax - and possibly GST as well.
You will not receive superannuation unless you pay yourself
You will not receive sickness or holiday pay.

You do not need an accountant to set up an ABN... you can apply online.
But you should get some information from the Tax department about the GST threshold.
If you don't charge GST - then you can't claim any of the GST on the items you purchase for work.
If you do charge GST, you can claim the GST credits from items you purchase (computers, stationary, travel expenses etc).... but you will have to lodge a BAS statement, and that can be daunting to do yourself... and expensive if you get an accountant to do it.

The ATO are very helpful if you want to phone them and ask advice on setting something like this up. They can tell you all your legal obligations.

It would be so much easier for you if your Step-Dad hired you on a casual pay basis so you wouldn't have to worry about all the tax implications. But then he would have to pay you superannuation (no holidays or sick pay though).
You really should charge a HIGH rate - higher than casual rates - to cover all the expenses and time that you will incur by doing this.

If you don't charge a high rate - you may end up regretting that you started it and spend more time with all the bookwork and taxation than it's worth.

Good luck with it though!
If you do become a contractor - you could look for other jobs to make it worth your while!