Ok, then...
To answser your first question, I'll refer you to
this article
To answer your second question, you need to know the voltage drop across the LEDs that you plan to use (Vled), the voltage that you're going to supply (V), and the current that the LED needs (Iled).
The value of the resistor can be calculated as (V-Vled) / Iled. With that number in hand, you'd pick the next higher up resistor value.
I hope that answers your questions.