If I know specific time, t0, how can I calculate RA of Greenwich?
I think you mean to ask, "What is the local sidereal time of Greenwhich?".
To calculate the GMST (Greenwich Mean Sidereal Time):
Right Ascension is a celestial coordinate, not a terrestrial one.
RA is used to locate a star on sky, and is measured between a fixed point in the sky (the crossing of the Ecliptic and the Celestial Equator) and the star's meridian.
It corresponds to terrestrial coordinate Latitude.
Thus, Greenwich has no Right Ascension.