Welcome to the forums and be prepared to read read read. This is what you should do second.
The first thing you should do is figure out how much you want to spend and what you want to do with your carputer. All the different areas of the forum are seperated for you to make SEARCHING more easy. In fact your two questions are answered many times over in the diferent forum areas. For the "were do i pulg in the power cable????" question look in the power supply section. You could use an inverter or another solution like an Opus. The second question could be answered in the input devices and/or lcd screen section.
Once again welcome and good luck.