First, what is your supply voltage.
Leds the colour of the led determines it's driving voltage.
White led requires 3 volts to light it.
If you have a 6volt dc supply , you can connect two in series which gives three volts across
There is quite a lot info on the Web about this and the use of resistors to drop the voltage required by the led.
It's using ohms law to calculate the resistor size but you need the find the spec for the led.
Led are polarity sensitive as well.