You need to register for GST if you have a turnover (your gross income excluding GST) of $75,000 or more from your enterprise, or if you expect your turnover to be greater than $75,000 in the coming year.
You are also required to register for GST if you provide taxi travel to your customers. Recently, the ATO issued guidance stating that participants in the ride-sharing economy (e.g. UberX drivers) are considered to be providing taxi travel.
This means that if you are a taxi driver or ride share driver, the ATO requires you to obtain an ABN (or Australian Business Number) and register for GST. The ATO requires you to start paying GST on your ride-sharing activity from when you first started earning this income.
You can obtain an ABN and register for GST through Airtax.