If you just want to get this done and aren't looking at making something fancy or plan on serious microcontroller work, I would just chalk this up as an Arduino project. Buy an Arduino and a FT232 USB UART breakout board from Sparkfun or wherever. I'm sure you could find a kaypad breakout as well. You can talk to the PC over USB using the FT232 breakout, though you would need to make the program that takes the input and does what you want. FTDI has drivers for this and some linux distros have support built in. There are plenty of Arduino tutorials out there for you to follow to read buttons and send data over the UART and that's about all you need.
You'll need a transistor to drive the relay with a flyback diode for protection. It wouldn't surprise me if there was a simple breakout for this somewhere as well.
Arduino doesn't really have proper power protection for automotive work, but I've seen plenty of them used without issue.
If you don't want to use an Arduino and want to use a regular microcontroller, you have some work ahead of you. Pick a microcontroller and try doing some beginner tutorials first to get into it at least.