The counting of the floors is continental - meaning that there is prizemlje (at-the-ground-ness) and spratovi (strata - the levels). So there are ground houses, i.e. no storeys, and storeyed houses, which have the ground floor and storeys upstairs.
Practically, the American 1st floor is prizemlje (at-ground-ness), i.e. ground floor. The 2nd storey is 1st storey (or, if none above it, just "sprat" - "where is the bedroom?" - "it's on the storey", i.e. upstairs). Then further on 3rd Am. is 2nd here... 13th is 12th etc etc.
In an elevator, the buttons are marked accordingly, and the ground floor is usually marked "Pr". What may be confusing is only the series of buildings in Novi Beograd where the elevated terrace counts as high ground floor, and the garages below it as low ground floor.