When I moved from Australia to the USA I went straight to an ATP, the written test prep I used was the Dauntless brand app.
There’s also a couple of other prep apps out there that work just as well.
Make sure you book your written through the CATS website.
You will also need to get your Australian license verified by the FAA which involves sending in a form to the FAA, then paying CASA a fee to do their part. The letter or verification is good for 6 months to get your flight test done in the sim. CASA as expected takes a while to send the verification back to the FAA so plan accordingly.
Make sure your Aussie license and IPC is current in every respect in addition to meeting the FAA part 61 minimum experience requirements.