We've tried to make this as easy as possible for newbies like yourself:
If you guy the OS, we can even install it for you if you so desire.
As for the rest of it, I would suggest utilizing the FAQs to help you understand what you're about to get into.
As for having it all amplified - you won't be able to have a computer that provides amplification enough for car audio speakers. You'll either need to use a head unit, your factory amplifier, or an aftermarket option.