What value resistors should I use for this LED?
This is running on the assumption that you are using a supply of 5v.

For the resistor calculations you want to use Ohms law V = IR, where I is the current you want to flow through the LED and V is the voltage difference between what you are supplying and what the LED is specified for. The voltage you want to use is the typical voltage listed on the datasheet, so for the case of red you have (5v - 2.1v) = 20mA * R -> R = 2.9v/20mA = 145 Ohm. The green and blue channels are calculated similarly using a voltage of 3.8v and 2.7v respectively.