First off, don't panic: most likely, your car isn't spinning around! There are a couple of common issues that can interfere with the Dash compass.
The first possibility is that your compass isn't calibrated. To calibrate, slowly move rotate your phone so it can get its bearings. Make a few rotations with the phone in a various positions - held upright, on its side, etc. Usually doing this three to four times will take care of the issue.
The other possibility is that you're using a magnetic phone mount in your car - unfortunately, the magnets in the mount will interfere with the sensors in the phone. The Dash compass won't work well in this kind of mount, so you'll need to find another solution for holding your phone if you want to use the compass.