Long before coming over you need to go to
www.faa.gov and download the foreign pilot verification form. If you can't find it send me an email and I'll get it to you.
You have to submit this to the FAA local office (FSDO) in the region where you intend to apply for an FAA certificate long before you actually plan to come here.
When you get here, go to the FSDO (flight standards district office - pronounced "fizzdo") in the city you are in, and they will issue you a restricted private certificate (no night vfr/ifr etc..)
You then need to prep for the written tests for Commercial/Instrument etc.. and pass those. There are ways to do that while you are still in Australia. check out
www.webexams.com for free practice tests. You can buy study guides from sportys.com (Gleim, ASA etc...)
Or you can come to my school in Chicago and we run a 3 day class to prep & pass the written tests.
www.americanflyers.net
You then have to learn the maneuvers (a few hours of flying) and take flight tests for commercial/ instrument rating etc...
Let me know if I can help. I'm an aussie working in Chicago.
Weasil