Belgrade Nikola Tesla airport is 15km away from the centre of Belgrade. There are two places you can get on the bus: one is next to Terminal 1 entrance and the other is in front of the Arrivals entrance.
Buses run every 15 to 45 minutes and the journey takes an hour and a half.
Every 15 to 45 minutes you have a bus from Belgrade to Novi Sad. Are recommendation is to use buses wich are travel only via Highway to Novi Sad.
You may come from Budapest to Novi Sad directly by Bus. When you arrive in Novi Sad use City Bus or Taxi.