You can take the Skybus which is $18 each way, and they say takes about 30mins each way.
You can take the Smartbus 901 which is $6 for the Myki card + $4 to the city = $10. Return to the Airport is only $4 after that. Take SmartBus 901 to Broad Meadows Train Station, then take the train to Melbourne city. Travel time 45-60mins. Make sure you have cash to get the Myki card from the bus driver and to put some money on the card.
Also, you will need the Myki card for all public transportation in Melbourne anyone.