For his/her first lessons, a new rider will need only three things – long pants with some stretch, footwear with a firm sole and a low heel, and a proper ASTM-certified riding helmet. An approved ASTM helmet is a must have item and you'll need to purchase one designed for horseback riding – bike helmets and hockey helmets are not appropriate for equestrian activities. Do not consider used helmets either, as there is no way to determine if there is structural damage inside the shell. Go to a local tack shop and have your new helmet correctly fitted by a trained salesperson there.
A Child's Safe Start to Horseback Riding A great article from Practical Horseman
Horseback Riding Helmet Safety thanks to Horse Council BC for this great brochure!