clifford, you need either a myki or (for country travellers) a V/Line paper ticket such as a single (one way) or return long distance ticket or a Family Traveller ticket.
See
Home - Public Transport Victoria and (for country travellers)
V/Line - Regional public transport for Victoria
You can purchase a myki from the 901 bus driver and you can also top up (or put an initial fare) on the myki through the bus driver. They do not give change. There is also an hourly metropolitan bus to Sunbury (handy if you live in Kyneton or Bendigo, or even Swan Hill or Mildura, as Sunbury is on the Bendigo, Echuca and Swan Hill railway line) as well as irregular buses to Moonee Ponds (but in most cases for the latter it's best to get the 901 and change at Broadmeadows to the Metro train.)
For instance an adult myki smartcard is A$6; the minimum fare is for two hours based on touch on times (i.e. when you place your myki on a validator on board a bus or tram, or at the entrance to a railway station) - sometimes at the platform, sometimes at top of any stairs or across a concourse and is A$3.76 so a $10 note or coins should do the trick nicely for myki plus an initial fare. Once you are on board the last vehicle in your two hour period you can travel (without changing) to the last stop so really it's more like a 'two hour 45 minute' validity period if you use it to the maximum.
The daily fare across all of zone 1 plus 2 is A$7.52 for adults: as many transfers as you like: bus, train and tram. This will get you to Healesville, Pakenham and Stony Point if you wish.
There is also a myki (blue) machine on the ground floor of MEL international arrivals, landside, near the cloakroom and also close to where one of the check in agents (DNata?) has lots of luggage usually stored. The myki machine accepts cash and cards. You still need to validate the myki on a bus, tram or at a railway station.
Touch on each time you use it (and for V/Line, which you may not use, ensure that you touch off at the station).