Iguanahead is right - use the site to get all of your info, but also look out for the seat sales - you can get some bargain fares.
Jet* is the best option from SYD. Although Virgin Blue is an alternative the flight times aren't so good and you'll need to change in Brisbane.
Jetstar* will allow you to reserve your seat on booking (although you may need to go back into the site to do this as an amendment). Choose an A seat - a/c usually overfly the island and make a tight LH turn to land from the north. You'll get a grandstand view from up there (or only of the sky from an F seat).