not going to an American uni as undergrad, but I have researched it as I wanted to before, not now though
If you really want to go, you have to be serious, as it is a bit costly to study there. If you want to go to Yale, Princeton, Ivy League (harvard, MIT and others) it is even harder to go into, but is doable if you have a good school report from year 9 to year 12, have done the SAT (or ACT) which are college admission tests that people in the US take as a standardised assessment (like HSC there but you can retake the tests over a few times in the year if you want to improve your score). Those uni's usually require you to take the SAT or ACT with a writing component (already in the SAT) and take 2 additional SAT subject tests (e.g. Maths I, Maths II, languages, sciences, english literature, other subjects/tests etc). The SAT and ACT tests are a little different. The SAT may be a little harder, it has english comprehension in multiple choice (think ICAS English tests) that test your vocabulary, punctuation, grammar, comprehension of passages and writing (however their writing is different from HSC English, not literature focused, more on writing to a prompt e.g. "American schools should give more students scholarships, do you agree" and they require about 2 pages of writing to get a good mark, along with a strong personal opinion and substantial argument throughout, kinda like a speech in an essay lol). It has a maths compenent which is really easy, basically HSC Mathematics level (no calculus, exponentials or logs, just basic trig, area and not too hard topics).
The ACT may be seen a little easier and is a bit different. It has a multiple choice for science, which is a bit related to the Year 7 to 10 content here. It has an english component that tests grammar, punctuation, vocabulary but I don't think tests english comprehension as badly as the SAT does. Their math component is more or less like the SAT math component, easy. In both the SAT and ACT you can use graphing calculators, and they supply more formulas that here in HSC maths you're supposed to remember. The ACT writing component is like the SAT writing component.
If you are looking to do the SAT subject tests, Maths I and Maths II are both comparable to HSC Extension 1 maths, however with topics that are not taught here (such as matrices and maybe some others). The Sciences offered are like the HSC sciences here content wise. Languages test about 2-3 years taught in that language if you take it here in school. Not too sure about the other subjects
The ACT and SAT cost money to do it here, about $100 or so for each test, subject SAT tests I think are around $60 or so each. You can retake the SAT or ACT if you want too, but it costs money again lol. The Sat is offered here I think about 4 times a year, May, September, March and January I think
The best time to do the SAT is in year 10 or 11, you can do it year 12 but if you do, try to do it in January so you can retake if you need to. Uni applications usually open in November of that year. Keep in mind if you do want to go to an American uni, you need to take a gap year as you need to complete year 12 then go overseas, as the US school year starts from September/August to May/June, and while we finish in November/December... yeah you need to take a year off then go into uni there.
If you aren't looking at top unis there in particular, you will still need your SAT/ACT and school reports, however they may not need additional SAT tests to be sat. All unis in the US really look into your cocurriculars, not just academics. And going to the top unis there is really competitive, so it helps to do as many cocurriculars/leadership/volunteering along with good grades for a chance. I know of two people from my school that went to Harvard straight from year 12, however one was a school captain and the other played sports competitively on a national/international level. I know of one that went to Yale but I'm not too sure what she did.
If you have any other questions, just ask