I thought that the light would have been just less intense but no, is not glowing at all.
LEDs are different from incandescent bulbs: they won't light up until the voltage gets up to a threshold that's pretty close to their normal operating level. Then the brightness goes up dramatically with very small changes in voltage.
Another difference is that they're not as tolerant of excessive voltages as bulbs: 15V will make a 12V bulb brighter and hotter, and burn out somewhat faster than normal. But it can be enough to make a 12V LED string into a "flashbulb" that only lights up for a fraction of a second. So make sure your 12V supply really is 12V (a lot of cheap ones don't regulate their output voltage).
If you look on this page
, you'll find that a lot of the questions you'll be thinking up as you learn more have already been answered.
For this case, you should just need a simple 10-cent NPN transistor to switch the "ground" side of the LEDs. Check the examples in the Playground area for driving higher-powered LEDs, and for driving small relays.