dimming leds and switches
LED dimming is best achieved through Pulse Width Modulation of the power being supplied to the LEDs. A quick web search reveals several PWM dimmers available, specifically designed for LED strip light dimming, e.g. this one on Amazon for $8.50:


Before buying and hooking up a dimmer, though, it might be useful to read a guide like this one, which will help you identify which specific rating of dimmer best suits your needs, both voltage and amperage.

It is actually fairly easy to build your own PWM dimmer, and that approach is more in the spirit of this site, than a buying recommendation.