I am trying to do some implementation on Xilinx ZCU111 RFSoC board. I am basically trying to send an OFDM signal and loop back to the processor and see its spectrum. I am btw using MATLAB's Avnet RFSoC explorer to control the DACs and the ADCs. I could successfully get back the OFDM signal at some instance but I could do it only by hit and trial. Can anyone here explain me exactly how to choose the ADC sampling rate and the NCO frequency in order to get back the signal at a particular center frequency? 

In general, ADC sampling rate is function of signal of interest BW.  The larger signal BW, the higher ADC sampling rate, Nyquist rate.

It depends how precise you want to read your signal of interest. Typical that should do the jobs in general case is for example 50 times higher sampling rate that signal of interest. In not so precise situations 2-4 times higher can also be enough.