lol you dont need to join a frat. you'll probably also hate frat boys. i thought they'd be like jocks, which i can tollerate. but they're all about the 'i went sailing with my girlfriend last summer in spain. look at my ralph laurent polo shirt. it cost $500. look at my boat shoes. i am great.'
however, for the first few weeks of uni, frats open themselves up during rush (to get pledgers), which means open house, open bar, etc
The drinking age's enforcement is relatively heavy. However, you can go to flight centre and ask for a student discount card. On the app form you can put a fake D.O.B. - they won't ask to see your real one, and even if they do, they'll still put down a fake D.O.B. Best thing to do is just keep the same birthday, but vary the year. I got into 80% of places in North Carolina (which has the strictest alcohol enforcement laws in the US).
During the first week (pre-uni) internationals will stick to themselves. This is mainly because hardly anyone is on campus, plus it's easier that way. Soon, though, especially when classes start, this changes. I kept hanging around one aussie guy (out of about 40 international students, and 9 aussies) , but apart from that, I hung around Americans. As an Aussie, you'll be somewhat a celebrity.
Make sure you also get facebook, as everyone has it in the US. Add some random people from that uni through facebook (and myspace). Make e-friends with them, and ask if they can show you around, go shopping with you, etc, when uni begins.