When you select the music thingy of your choice, just read the box to make sure its compatible with is car. You'll also need to purchase the electrical harness for his specific car.
To install, you'll need detailed instructions on how to dismantle the dash. Some cars are complicated, and some are easy. Instructions are usually avaliable on the internet, just be sure you get the exact make and model. The harness installation kit should have detailed instructions on how to connect the wires.
You'll need a soldering iron, and solder to get the wires together. If you haven't soldered before, you'll probably want to watch a few youtube videos, and practice a little bit.