To use an Arduino as a smart switch (we'll discuss that in a minute) you need some "electronic switching" capabilities, a transistor circuit, maybe a mechanical relay will be enough, all depending on what you are switching, the current in draws and the voltage of course (well, 12V in your case).
So it looks like you are way to far from solving the problem and for that matter you would need to know more about electricity, electronics, and programming.
As for what you may need, it should start with what you want, and even that is not clear, why you need a solution based on a microcontroller in the first place.