How would someone from Australia apply to American Colleges/Universities?

I’ve tried to look this up on the internet but I’m so totally confused. I was wondering if someone could please explain your system to me in a dumbed down language? Thanks :P
I’m in my last year of high school in Australia, I will finish in December of this year. If I wanted then to go to America, what do I do? What’s the difference between College and University? How do my scores here change to suit your scores? When would I have to apply? Do I have to sit some kind of aptitude test, like what are they… SATs? What is GPA and GPA scale? Are international scholarships offered?
Anything you could tell me would be greatly appreciated, thanks :)

edit :: I’m looking into Engineering, specifically aerospace engineering. Any schools you would recommend I look into?