I'd start off by listing out your questions on a piece of paper next to your computer at your desk. Go through them one by one and search on these forums for the answers. They are all here to be had, expecially the questions you've asked.
If you have really searched like you say you have, then you would have found the threads that provide the answers...even the FAQs for them.
There's an entire sub forum dedicated to PSUs for the car and another for GPS. There's FAQs concerning how to get your computer audio to your system as well. It's all waiting for you to find it. People on here don't come back just to spoon feed others, they come back to research and ask more questions after they've done the respective research. I suggest you do the same.