It is very important to have a helmet that fits properly and does not block your vision or hearing. If you ski where it is quite cold, a helmet with vents that close will be important. If you ski in areas where there the temperature can vary greatly during the ski day, the ability to open and adjust the vents is important.
Stop by your local ski store and try on a variety of helmets to see which brand fits you the best. The staff at these stores is knowledgeable and will assist you in finding one that fits your head the best.