What is the voltage rating on the LEDs?
What is your source voltage?

Typically, an LED has a 2 to 3 volt drop. You can get LEDs with different voltage ratings, they are usually either clusters or have a resistor in series with them.

The easiest method is to buy a potentiometer and dial in the brightness you desire.
Mathematically, measure the resistance on the LED (forward).
I=V/R (use this to find the theoretical current or your target current. R that you measured on your LED, V at its rated voltage)
RR=V/I - RL where RR is your resistor, V is your actual voltage, I is calculated above, and RL is the resistance of you LED.