What power do I need for my LED string
To do this, you want an LED driver that has a 12V output (easiest to find). Then looks like you'll need 18W (3W * 6 strings). You /can/ wire them in series but parallel would be easier. On Amazon, Meanwell makes a 12V 5A driver that is more than overkill but will work. 

On a side note, unless internal circuitry is involved, LEDs are D.C. devices, not AC. Also, for strings of LEDs, you want a constant current driver. The brightness of the LED depends more on current through vs voltage across.