Useful information for people new to Melbourne.
Getting to and from the airport is typically done via the SkyBus airport shuttle. But it's expensive (like twenty-and-a-half bucks). It goes express between Tullamarine and Southern Cross Station.
But you can get a regular bus (so you can use a Myki [see below]) that connects with the regular train system. It's a bit less straight-forward than the SkyBus, but if you're confident in navigating the PT network, it's a great way to save some money. Also great if you live / are staying on a train or tram line that the bus connects to, because the only place the Skybus stops is at Southern Cross Station.
The buses all leave from outside Terminal 4, and are:
If arriving from the airport you can either buy a Myki at: