Chose an LED current around 5 to 10 mA. From the graph of forward voltage versus forward current for the LED you are using, read off the forward voltage.
The resistor value may now be calculated from (Vsupply-Vled forward voltage)/chosen current.
The nearest preferred value is fine.
220 ohms is a pretty good standard value to use.
Note that there are additional issues when you start to do more serious things. The forward voltage drop of an LED depends on the manufacturing method(colour). The intensity varies widely with current (efficiency and how the human eye perceives colour). You may also have to consider total current consumption in your system, and the processor output port can only supply so much current before the voltage on the pin starts to drop.
Cheers,
Richard
