If you are a visitor to Melbourne or parts of Victoria and you want to use public transport, you need a Myki (money) card. You can buy these from machines around the city at tram stops, at railway stations and from newsagents and convenience stores for the cost of $6 for an adult card or $3 for concession and seniors.
Put some credit on the card when you buy it and touch the card on when you board a train, tram or bus, and touch it off when you leave a train or bus. There is no need to touch off when you leave a tram as the fare charged automatically defaults to the minimum payable*. That is all you need to know really. You don't need to know how it works. It just does. Make sure you hold the card still and not look like a tourist by waving or moving the card all over the place.
How much money to put on your Myki? Adult $25 will last you three days and allow you to travel on public transport each day for the day. $12 would be enough for people of a certain age for the same.
Ok, you want to know a bit more. The fares that will be deducted will be either a two hour fare or a daily fare. It does not matter which form of public transport you are using. The system is inter connected for all trains, trams and buses and within the two hours or within the day, it does not matter how many times you use public transport or what type of vehicle (note that, Sydney).
These prices are for an adult and seniors or pensioners are correct at this date. There is not a concession or senior's fare for overseas visitors excepting child fares. Interstate visitors with a State Seniors Card can buy concession tickets as can interstate age pension card holders.
Full fare first then concession and seniors fare for travel in Zone 1 which covers an area of about twelve to twenty kilometres, or seven to twelve miles, radiating from the city centre.
2 hour $3.58 and $1.79
Daily $7.16 and $3.58
Weekends and public holidays:
$6 capped full fare and concession as above. Victorian seniors travel free on weekends. Interstate seniors pay concession fare.
* It becomes a little more complicated if you only travel in zone 2 on a tram, but few visitors to Melbourne ever will.