Hello and welcome to DefenceTalk!
I'm an American, so my depth of knowledge here may not be as great as the native Australians or any American expats with ADF service here (if there are any).
Before you read the rest of this post, I encourage you to read this
Why the military?
This is the question you have to ask yourself before you go any further.
Joining the military of any nation and
becoming a soldier is one of the greatest challenges you will ever face in your life. In boot camp/basic training, you'll get torn down to the ground and built back up again as a solider.
You'll learn lessons that you will never forget and become a new and very different young man or woman. You'll give up some of the best years of your life for a higher cause. You'll be hardened in a fighting machine, and no matter what job you do, ultimately what you do contributes to end result of any army: killing, maiming, and destroying other human beings to protect your family, your buddies, and your nation.
Now sometimes, those hard, those ugly realities, they're justified. As George Orwell said "we only sleep peacefully in our beds because rough men stand ready to do violence on our behalf." But you have to decide
for yourself that that is something you're willing to do.
I applaud you for even considering the armed forces. It shows that you are a young man or woman of maturity, drive, and focus, and for that I salute you. As a soldier you'd be joining a proud tradition and joining a line of service that I too aspire to join.
But before you do anything else
you have to decide that you truly want to be a soldier.
Next, why the ADF?
I don't want to sway your thinking; but as a American you have the chance to join the US Army, Air Force, Navy, or Marine Corps, four of the best-trained, best-equipped, and best-paid military forces in the world. You'd have a whole new set of opportunities you wouldn't get in the ADF, including greater chances for travel, faster promotions, good benefits and more educational opportunities.
Plus, as an American, you can walk into a US Army recruiter's office and be enlisted in a much simpler process than the on you, as a foreign national would have to take to enter in the ADF.
If you want to join the ADF, it will be a long, difficult and at times, discouraging road. In addition to the hurdles of military life, you have to go through the bureaucracy of becoming an Australian citizen. But if you decide that the ADF is your one and only choice, then Bravo Zulu* for you!
I genuinely wish you the best what ever choice you make!
*US Navy slang: "Well done!"
End this part of this post
Now, back to your original question.
There are several routes which a foreigner can join the Australia Defence Force (ADF), each of which is determined by your citizenship status and current or former military service.
If you currently have not served in the armed forces of any nation and do not have Australian citizenship, you can't join the ADF.
However, as an American non-veteran, there's two things you can do to change your status.
1. Apply for Australian citizenship and then, once you have this, enlist. There are several ways to do this. You could a) move permanently to Australia, although before you do this you would need to file immigration papers (permanent visas, etc.) b) marry an Australian. or c) if one or both of your parents are Australian, then you're already a citizen and can join up more or less right away....
I'd suggest looking at the Australian Dept. Of Immigration and Citizenship's website for more info.
2. Join the US military. At minimum, you'd need to reach the rank of Sergeant (a pay grade of E-5) to be eligible for an enlisted position in the ADF (you'd enter the ADF as a sergeant). You'd also need to have a US High School diploma or its equivalent, although you'd need this to join the US Army or Marines in the first place.
Once you've reached this rank and have decided the ADF is still for you, let your US enlistment expire, get Australian citizenship, and enlist in the ADF.
Any idea what kind of trade/career you'd like to pursue in the Australian Army? Different jobs are more competitive to get into than others, and the Army is going to recruit based off of its needs.
The ADF's overview for overseas applicants (As an American you are classified as an "overseas applicant")
Recruitment Centre | Defence Jobs
The Australian Army's site for overseas applicants
Recruitment Centre | Defence Jobs
The ADF Website regarding Australian citizenship
Recruitment Centre | Defence Jobs
The Australian Department of Immigration and Citizenship
Australian Citizenship – Becoming an Australian Citizen
Sorry for the length of this post. I hope it helped in some way.
You asked a really good question and I wanted to give you a thorough and complete answer.