Belgrade city public transport is provided through a network of bus, trolleybus and tram routes, as well as Beovoz - City Railway

PlanPlus - routes of all lines with all stops (in Serbian only) - Android App is HERE
Moovit App (Android and iOS) - Live directions with get off notifications with alert when it’s time to get off.
BusPlus Android App - allows you to get information on public transport lines in Belgrade and stops from your current location in real time. The application also allows quick search of the desired stops and their location on the city map, and also shows the placed vehicles in the daily traffic for the selected lines or all the lines that encounter a particular stop.
SMS servis- By calling * 011 * xxxx#, where  'xxxx' is the code of the station (highlighted at each station), travelers can freely find out how many stations away are the vehicles that reach the station where they are.

Paying

If you jump on a trolley, tram or bus in Belgrade, you will initially be taken aback by the fact that nobody seems to be paying to use the system. Don’t be fooled into thinking this means you can ride the cars for free though, as you’ll only find yourself on the end of a fine. Ticket inspectors are more prevalent than most assume, and more often than not they will be in plain clothes.

To buy a ticket for the system, you need to head to one of the many kiosks or points-of-sale dotted around the city and purchase a ‘Bus Plus’ card. The system acts on a ‘pay as you go’ basis, with one 90 minute journey costing 89 RSD (for 1st or 2nd public transportation zone). There are the following types of tickets:

Paper contactless cards  - 89 RSD: if you are not frequent user you should buy this card, must be authenticated (validated) at the beginning of the trip (check in).

Paper tickets from the driver - 150 RSD: individual ticket for one ride,

Non-personalized smart card - 250 RSDtype of electronic wallet. The user card complements the desired amount of money, each achieved a run-off card with the amount that represents the price of the ride. These cards are not time limited and may subsequently be amended at all locations for sale. Non-personalized card must be validated when entering the vehicle (check in). The maximum amount of one transaction is 2,500 RSD, and the maximum current amount on the cards is 5,000 RSD. For each transaction (additional fee) of 900 RSD or more, additional 100 RSD will be entered on the card of the user (for example, for payment of 950 RSD, 1050 RSD will be entered on the card).
With this card, it is possible to pay a ride for more people, before validating, select the "group card" option and enter the number of people.

1 day paper card -  250 RSD

3 days paper card - 700 RSD

5 days paper card - 1,000 RSD

Payment by Master PayPass card - 98 RSD: Immediately upon entering the vehicle, bring the Mastercard PayPass card closer to the validator, to ensure that the validator accepts payment. The Mastercard PayPass card should be read on the validator at each change of vehicle (one drive, 98 RSD, will be charged for all rides within 90 minutes). With Mastercard PayPass card it is not possible to select group tickets.
When checking the ride payment, it is necessary, at the request of the Ticket inspectors, to bring the Mastercard PayPass card to the control device in order to verify that ticket has been paid.