Well as far as I know there aren't that many (as in no) good dive sites around Melbourne. I have only been scuba diving twice, both times in Queensland diving the great barrier reef (just diving a few metres down with a dive master to look at the reef). If you want to find out some general information about getting certified
http://www.padi.com/ is the place to go. I hear it's not difficult obtaining the open water diver certificate.