So, the name of Air port is Nikola Tesla. Bus station or railway station are one to another in down town.
Taxi from Nikola Tesla to down town is 10 - 20 Euros. If you are coming with bus, no problem, on every 50 meters, there is hotel, hostel, or room.
The bags are stored and walking is starting.
Where to go is first topic? The correct Answer is Knez Mihailova Street and Kalemegdan fortress.