Ok, because our school actually had people come from the American embassy talk to us about American University.
Part of the requirement for most American Universities is the SAT, which is administered by the College Board (I have a friend in my Spech class planning to sit this exam soon, he is in Year 12). Most American high school students sit the SAT in Year 11.
They also look at other criteria for example your VCE, if you do any APs (you won't) and often a written application. The process is also quite decentralized (notice the American spelling) in that where you'd apply to each university that you want in America (or wherever your sports scholarship will take you) rather than here in Australia where you'd apply via VTAC.
For physio, first of all, physio is a post-grad degree in America, you can't enter the course without a bachelor's degree. Secondly, next year, you will need to sit the UMAT if you want to be considered for a place at Monash (there are unis that don't need the UMAT, La Trobe being one of them). You won't have to worry about the GAMSAT as that is for med only (post-grad med)