Why does my car shakes a lot at the first 15 minutes after it get started?

Hey, I have a Hyundai Getz 1.4L 2006 Auto! When I start it up in the morning it shakes like hell and it keeps shaking for 15 minutes while I drive it and then it becomes as smooth as it was new!! I know it is very weird. And it does that every time I don't start it for more than 4 hours! When I put it on Reverse the shaking almost goes away! lol...plzz help :S