I've never used Contiki myself, but I hear it's fabulous for first-time travelers. They take you to all the touristy places you will want to see and organize it so you don't have to worry about a thing
I used a company like Contiki my first time to Australia and had a blast.
We used Youth Hostel Association (YHA) to plan our 7 weeks in Australia/NZ in 2002. They were great and cheap to work with, but we had to worry about a lot of details ourselves. The good thing about it was we got to see what we wanted to and spend as much time in certain places as we wanted. I'd definitely stick with Contiki if it's your first time, though. I hear they're a lot of fun, too