how to display speed on arduino?

so im going to put my arduino in the car, and i want it to display the speed of the car, how can I do that? does the arduino uno already have a built in accelerometer or do i need to buy a GPS module or something? (please include as much detail im really new to this and doesnt understand jargon) thanks

I have one of these GPS units from adafruit. Dead simple to use: they supply a library.

You'll need an LCD or whatever you want to display it on: look for an I2C one so it uses only 2 data pins (plus power of course).