I think you're totally out of your depth here. I assume that you're using this setup because these are the parts you happen to have.
You need to start with the load you're trying to control. What is the power of the lamps? What current will they take? Then select a contactor or relay which will switch that current and will interface easily to your control circuit. If you can, use a small relay; it'll be easier to drive than a contactor.
What is the resistance of the coil of the contactor/relay? I'm assuming it's a 12V DC rated coil as you mention a 5V to 12V converter. Knowing that, can then work out how much current is needed to drive the coil.
Then design the interface between the microcontroller and the relay/contactor. Using the output of a microcontroller to supply a step-up converter won't work; the microcontroller won't be able to produce enough current to drive the converter. Once you've sorted out a supply for the contactor or relay, you'll probably have to use a bipolar transistor or FET to provide enough current to drive the relay from the microcontroller.
Since this is switching mains voltages, I suggest you try very hard not to kill yourself.
