Subtract the IR LED forward voltage (Vf) from the power supply voltage. That gives the voltage that the resistor must drop (Vr). Divide the difference (Vr) by the required LED current (insure that the current is less than the max continuous current from the data sheet). The result is the resistance required. Pick the closest standard value.
That is for a basic IR LED. Like bigred1212 says, your particular LED could be different.