Watts = Amps * Volts
100 watts = x amps * 120 volts
100 watts / 120 volts = ~.83 amps
Or you can manage the amps by testing it with a test equipment.
Here you can do this by watts dividing by volts.
Here is a formula :
Try this ohms law calculator app through which you can easily came to know about the calculations of voltage,current and resistance.
Hope that helped..!