Helmets must fit snugly, but not painfully tight. While holding the chinstraps outside of the helmet, pull it down over your head, taking care that the chinstraps do not go up inside the helmet. A proper-fitting helmet will feel snug with fairly even pressure around the sides and top of your head. On most modern helmets the liner is designed to be a little extra snug at first, but will break-in after several hours of riding to match the particular shape of your head.
If you are not familiar with how a helmet should fit and feel, be sure to ask for assistance from a salesperson or a more experienced rider. As a rule, the helmet should not be able to move around on your head without pulling on your skin. A loose fitting helmet is not only dangerous, but may also be noisy and tiring to wear. Most people make the mistake of buying a helmet that's too large.