At 20mA its forward voltage is 1.2V,

at 100mA its forward voltage is 1.4V.

So, lets say 20mA, with 5V supply:

Voltage across LED = 1.2V (from datasheet, assuming the current is set right)

Voltage across resistor must therefore be 5-1.2 = 3.8V

Current through resistor is 20mA,

R = V/I by definition of resistance, so R = 3.8/0.02 = 190 ohms.

Arduino pins can handle 20mA, and have roughly 40 ohms of internal resistance, so an external

resistor of 190 - 40 = 150 ohms would be appropriate.

At higher currents you'd need some sort of driver transistor, or MOSFET or relay or whatever.