just use the resistor calculator in the pinned topic
the controller voltage is 5V, LED's draw up to 30mA
provided you use a high enough value resistor the LED won't pull to much current and blow itself up!
picture an LED as a tap, thats stuck full on
picture the resistor as a piece of narrow pipe
when the taps full on, it sprays water everywhere, the only way to reduce the water flow is to prevent so much water getting to the tap
if you put a piece of narrow pipe in, less water can now get to the tap, controlling the water flow!
in simple terms the resistor 'blocks' electrons, if an LED has too many electrons it blows up