The explanation of Ice Damming is fairly easy. However, it's prevention is not cheap in most cases. A little research will get you a long way. Maybe you only need a ridge vent if you have soffit vents. Maybe you need to un-insulate if the roof is insulated. Maybe you need soffit vents and a ridge vent.
Go to My Favorite WEB Search Engine, and enter in +"prevent ice dams" (double-quotes and all), and press Search. You'll get a number of 'hits' on the subject. After reading this, if you have more specific questions, do start a NEW post.