Hint: you really don't need one of those on-line calculators. It's a very simple computation based on Ohm's law:
R = (Vtotal - Vled)/I
Translation: the resistor value is equal to the (total supply voltage minus the LED's forward voltage) divided by the current.
Example:
Vtotal: 24V
Vled: 3.6V
I: 100mA
therefore R = (24-3.6) / 0.1 = 204Ω. Use the closest standard value (either 180Ω or 220Ω depending on whether you want to go a little over or a little under on the current). Remember: less resistance = more current.
But: that's only half the battle (calculating the resistance value). You also need to size the resistor in terms of how much power it will be dissipating.
P = E x I
(translation: power = voltage x current)
Since we have 20.4 (24-3.6) volts across the resistor, and since the current through the whole shebang is 100mA (0.1A), the power is 20.4 x 0.1 = 2W. So you're going to need a 2-watt resistor. (Doubt you'll be able to get it at Rat Shack; try going to a real electronics vendor. There's a "sticky" thread in the Electronics Chat forum with a list of suppliers.)