This is a common question, as so many of us want to fill all those slots on our characters as quickly as possible. Helms are generally only available beginning at level 24 (with the Azure Silk Hood, a Cloth crafted item). There are some that are available sooner that offer a marginal amount of armor, but these are usually only available during such holiday events as Hallow's End and Noblegarden. Shoulder armor is available sooner, around level 15, usually as drops from various monsters.