I have a variable resistor and i have no idea how to wire it?

The variable resistor will work for a single LED, but if you move beyond that, you should check out PWM circuits, mainly because multiple LEDs won't react the same given a certain voltage. The way you dim an LED is through Pulse Width Modulation. It's basically a control that turns the LED on and off very fast, and you vary how long the 'ON' portion lasts, so that the perceived light is less than if the LED was continuously on. Check the linked site for more info.