Since you are new to electronics, you may want to follow this tutorial: itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads It tells how to vary the brightness of a light bulb using a transistor. For your candle project, just use a light sensor (as recommended by hellonearthis) instead of the potentiometer in the tutorial circuit.
I just bought a 12 Volt DC bulb from a local auto parts store and soldered a couple of wires on the base. You could also buy a socket.