OK, that's NOT a transformer, it's a DC-DC converter - which is the type of thing you need.
At the bottom of that page it gives a link to a 20W version, that should be more suitable. However, you could really do with finding out what power the LED's require, do you have a multimeter you could use to measure it?.
On that same page there's also a link to MT3608, these also might be suitable, but require adjusting to 12V, so you'd need a meter for that as well. I've used a good few for various purposes, and they work quite well.