Every Wiccan is his or her own priest or priestess.
In traditional groups this title is bestowed upon people when they get their first degree, and indicates that they now know the basics of Wiccan theory and can perform rituals for themselves. However, since this is a movement with no central authority (and with no official initiation certificates issued), all people who consider themselves to be Wiccans can call themselves priests or priestesses, which is to say, taking on responsibility for their own spiritual development and subject to no ecclesiastical authority.
To be a priest/ess for oneself is not quite the same as being a priest/ess for other people. Being one's own priest/ess means that a practitioner has a direct, unmediated relationship with the divine, i.e. s/he doesn't need a middleman to make contact or to receive blessing.
When Wiccans are given a second degree by their colleagues, they are considered to have enough competence to conduct a ritual for other people, or at least to take the leadership role in a joint ritual, or to facilitate other peoples rituals and offer teaching. At this point they can call themselves high priests or high priestesses. In the traditional group, it was intended that everyone should take the second degree in due course, and become high priest/ess. In casual groups, or in the case of solitaries, many people are content with a single initiation.
Incidentally, the feminine form priestess is not considered lesser, in the minor leagues, or not-quite-serious, as the old, now discarded terms like poetess or authoress were. There is no move in Wicca to drop that ess as there has been in the world at large. Thats because in Wicca the word Goddess has great resonance. She is primary. The God is consort or lover or son, but never master. And in the circle, though the high priest and the high priestess are a team, it is she who has the final word.