Ok, so you have started a business….. the question you are asking.
Do I need an ABN and be GST registered?
You will need an ABN regardless, but do I need you need a GST registration attached to the ABN.
Yes, if your income is greater than 75k or you are a hire car or taxi driver. If your income is less than 75k and you are not a hire car or taxi driver then registering for the GST is voluntary.
In some cases if you income is less than 75K and you have reasonable cost associated with running the business it may be a better option to register as it will increase your profits.