You just want all 100 to turn on/off together?
Wire them up in series in strings of 5 with a current limit resistor (~150 ohm), from +12 V to the drain of an N-channel mosfet . You maybe be able to get away with 6 in series, depends on the forward voltage drop of the LED. Adjust the current limit R accordingly.
The gate will be connected to an arduino output pin thru a 100 ohm resister, and a 1K resistor to ground if you find it is not turning off satisfactorily.
The source will be connected to ground.
You will have 20 strings of 5+resister wired in parallel.
Choose a logic level MOSFET with low Rds. It will have about 400ma going thru it.
Here is an example but with a 20V source
You could also use a part like ULN2803 if you are not comfortable with discrete transistors.