If you're an American, driving in a foreign country can be a little intimidating. If it's Australia you're visiting you'll have a lot to adjust to, but not too much. Because both the US and Australia's primary language is English, reading signs will not be any problem, provided you speak and read English as well.
The first thing you'll have to get used to is driving on the other side of the road. That can be a little disorienting at first. You just have to remember that Australia's right turns are much like America's left turns.
The second thing you'll have to get used to is the interior of the car. You'll have an urge to get in on the wrong side of the car to begin with, but once you find the correct side, you'll have to adjust to where stuff is. If you are driving stick, be prepared to shift with your left hand. It's VERY strange, but you'll get used to it ever. Also, the blinker is on the right side of the steering column. Flipping the other blinker will actually turn on your windshield wipers, which while funny the first few times, it gets annoying after a while. The placement of the brake and the gas pedle is the same as it is in America, so not everything is completely different at least.
The difference in orientation of the driver's side of the car is actually a blessing in disguise. In America, as a driver you're always the closest side in the car to the divider. If you always keep that in mind you'll never have to worry about turning down the wrong side of the street.
Turnabouts … I never did absolutely master these beasts, but they're really really easy to deal with. The way I think of it is that when confronted with a turnabout, you only need to look right to be sure no traffic is coming. If a car is approaching, let them pass, then go. It's possible to do this without stopping if you pay attention ahead of time to see who's entering the turnabout and when. Also if the turnabout has two lanes, pay attention to which lane the incoming car is in. You may be able to safely enter the turnabout if the car is in a different lane.
Finally be patient with yourself and have a good time. No one is perfect, so anticipates mistakes, and meets them with humor. You'll forget the little things like where the blinker is, or that you need to down-shift occasionally. Sometimes though, you'll master these tips and profit.