while most people are going to tell you that this isn't the right place for this kind of question, and a CarPC is WAYYY over your head if you don't even know how to wire a switch, and that you should have read the FAQ and searched all over the place before posting... I'm going to give you an answer before telling you those things.
the easy way is to ground whatever device your connecting, and the put the switch in between the +12V source and the device... make sure you put a fuse for the correct amperage before the switch though, and try not to use bare stripped wire for anything to avoid fires. instead use quick disconnects for wire-to-wire connections and ring connectors for terminals at the very least.
the more proper way would be to also include a relay... but this is more complicated than necessary for most applications.
You probably want to check out a forum specific to your vehicle so that you can find a good +12V terminal in your fuse box, or at least a good circuit to tap into. This will prevent you from tapping the cigarette lighter, radio, or running wires from your engine bay. You may want to search around some FAQs on a car audio specific forum....
Now, that said, "A CarPC is WAYYY over your head if you don't even know how to wire a switch, and that you should have read the FAQ and searched all over the place before posting."
BTW, you should be finding a way to turn your amp on automatically with your stereo instead of wiring a switch.