How to start with creating a device?
You've got the general idea down already, so start documenting what you know - make a block diagram.  Draw the blocks (processor, battery, light switchy thingy), determine where you have to draw lines that connect them together.  Put a lot of '?'s all over the place for everything you don't know. 

Above all figure out specific questions!

For example, you know you need a door sensor - but what kind?  I just did the first thing I thought of - Google it.  Despite all of the results I know (from experience) that you'll probably want a door sensor similar to this or at least one that follows the same principles.  So read up on it, figure out how it works (in this case one side is a magnet and the other is a Hall effect sensor that detects the magnet) and then find more questions: how do I connect this to a microcontroller for example.  

You've got a lot of specifics and a lot of questions.  Write down and sketch out everything you know, figure out what you don't know, do research and think up specific questions to ask here.  Trust me - we get enough bad, vague general questions (fake example: "How do I make a robot that responds to my voice?") but we're more than happy to answer specific well-reasoned questions like "I have this Hall Effect sensor but I can't tell from the datasheet if the output is TTL or just an analog voltage.  What is the best way to connect this to a microcontroller to determine if a magnet is present?"

And sadly, expect a lot of delays and frustrations.  I tried making a Halloween costume this year and I'm still working on it for various reasons: I designed the reset circuitry for my microcontroller incorrectly so I couldn't program it until I ripped it all apart, I had the wrong PCB footprint for my contact switches so none of them worked, I didn't put an external power connector on the board so I could only work on it while I had a ready supply of 9V batteries, my PWM outputs cause so much noise on my accelerometer (analog) output lines that they're unusable, my accelerometer is so noisy I can't integrate its output to obtain velocity, my Bus Pirate needed a firmware upgrade so it wouldn't choke on my serial data, yada, yada, yada.  I may not be done by next Halloween!  You'll definitely need a lot of patience because these things happen and happen often.

Good luck.