A signet ring is traditionally worn on the little finger of the wearer's non-dominant hand. That means a right-handed person would wear the ring on the little finger of the left hand.
Signet rings were traditionally worn on the little finger of the non-leading hand to make it easier to create a wax seal (to seal a letter, for example). The process involved rolling the ring into the wax in order to make an impression of the ring's design into the wax itself. The ring's impression would be officially recognized as the signature of the wearer. Signet rings were mostly used by political leaders, religious leaders and wealthy members of society.