I'm also a beginner looking at installing an arduino in my car and controlling it via voice commands run through a Windows tablet. The programming is cake but I'm not sure of how to wire it. I know I can't just stick a wire into the power window switch or it'll fry the arduino board. I think I need a diode but I'm a hardware noob. Maybe we can help each other out on this.
Edit: You asked about how the computer will send the info to the arduino... I should mention that I am using an ethernet shield to create an arduino server. I think I should be able to create a private wireless network but haven't set that up yet. Using my laptop for testing, I'm hardwired into the arduino ethernet shield, sending commands as simple HTTP GET requests (e.g. http://arduinoserver:777/command?pin:9:on). The arduino parses the request parameters and supplies or removes power from the specified pin.