how do you get 12v of power sent when the arduino only sends 5v?
Alternatively, use one 12v wall wart to power the Arduino (via the DC jack) and use the "Vin" pin on the Arduino, which will then have 12v on it, to power your lights.
The vin expects some nice and clean 5v, put it 12v and you have a fried arduino.
The power pins are as follows: * VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin. * 5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply. * 3V3. A 3.3 volt supply generated by the on-board FTDI chip. Maximum current draw is 50 mA. * GND. Ground pins.