Look to the Circuit diagram shown. You want to calculate the resistance of the currentlimiting resistor R_{v}. According to Ohm's law...
R_{v} = U_{v} ÷ I With Kirchhoff's Voltage Law (KVL) you can calculate U_{v}... U_{v} = ( U_{0}  U_{LED} ) and calculate the currentlimiting resistor R_{v} of the LED... R_{v} = ( U_{0}  U_{LED} ) ÷ I 
Assuming the maximum forward current is 20 mA, you might want to limit the current to 15 mA. Hence you calculate ... R_{v} = ( U_{0}  U_{LED} ) ÷ I = ( 9 V  2.0 V ) ÷ 0.015 A = 466 Ω 

Since you can not find a standard resistance of 466 Ω, you use the next higher resistance of 470 Ω.
Next, you look at several LEDs in a circuit.