As stated above, you'll want to convert your potentials from the polarization experiment to RHE first. Your reference probably comes from the manufacturer with a potential vs. NHE (normal/standard hydrogen electrode), so you would apply that correction then there is a pH correction to shift from NHE to RHE. Thus you'll have to know the pH of your electrolyte. To be more accurate than trusting the manufacturer, you can calibrate your reference against a known redox couple. Once you have the potential vs. RHE, the overpotential is simply the potential past 0 V vs. RHE (theoretical). Overpotential is typically assessed at onset and at a given current density (most commonly -10 mA/cm2).

Theoretical potential of the Hydrogen evolution is  0.00 V w.r.t. RHE (reversible hydrogen electrode)....

You have to go through online what is RHE scale, later you will find out your overpotential by subtracting the theoretical potential with your onset potential........But keep in mind your potential was corrected to the RHE scale.... 

As stated above, you'll want to convert your potentials from the polarization experiment to RHE first. Your reference probably comes from the manufacturer with a potential vs. NHE (normal/standard hydrogen electrode), so you would apply that correction then there is a pH correction to shift from NHE to RHE. Thus you'll have to know the pH of your electrolyte. To be more accurate than trusting the manufacturer, you can calibrate your reference against a known redox couple. Once you have the potential vs. RHE, the overpotential is simply the potential past 0 V vs. RHE (theoretical). Overpotential is typically assessed at onset and at a given current density (most commonly -10 mA/cm2).

You will get more accurate results by generating a Tafel plot than with a simple polarization curve. Collect chronoamperometric data at several relevant potentials (I usually try 10 potentials at 20 mV intervals, running each for 5 minutes, and averaging the current density for the last 4 minutes). Then plot log(current density) vs overpotential (in this case, potential applied vs RHE). The overpotential for a given current density can then be caclulated using the curve.

Thank you very much , i got the point. Thank you