To calculate Sw of a zone of interest, you first need to calculate the resistivity of the formation water if it is not known
The archie method is the easiest:
Rw = porosity^2 * Rt / a
Rt is the resistivity of the uninvaded zone and is your deep resistivity reading, not affected by flushing of drilling mud.
a = 0.81 for sandstone reservoirs and 1 for carbonate reservoirs.

With your Rw value (either from calculation or water catalogue):
Sw=[(a.Rw)/(porosity^2.Rt)]^1/2