Do you mean in the general assembly hall? It's by alphabetical order by their English name. Each year the starting point is rotated annually by the Secretary-General via ballot draw.

https://www.unitar.org/ny/sites/unitar.org.ny/files/UN_PGA_Handbook.pdf

page 18, which is 12 of the pdf

Follow-up question: how does the United Nations define and/or recognize a nation?