arduino voltmeter

For one of my first projects for my arduino, i thought i would try to implement a simple voltmeter. Basically i was thinking i would take advantage of the fact that the analog input is basically a voltmeter already. so what i need to do is figure out how to take a voltage on a rage, say -10V to +10V and map that to a output voltage of 0 to 5V. I was thinking i would probably do something with op-amps, but i dont know very much about them. could anyone help?