Wicca is a neopagan religion founded in the 1950s by British writer and occultist Gerald Gardner. Today, Wicca, which combines nature-focused spirituality with ritual magic and the worship of both male and female deities, is divided into a large number of different traditions, each with its own rules -- and many with a distinct lack of them. The leaders of Wiccan covens are called High Priestesses and High Priests, and although there's no single pathway to initiation as a High Priestess or Priest, there are some common elements.
Find a local Wiccan coven in your area. New Age, religious or occult bookshops may have advertisements for local groups, while many covens and Wiccan groups also have a significant online presence. Not all Wiccans organise themselves into covens; some practice "solitary" Wicca, either because they prefer it or because no coven is available to worship with.
Discuss the religion with other members of the coven. They may recommend some reading for you to help you learn about Wiccan teachings. Eventually, if you seem sincerely interested, they may invite you to be initiated into their coven.
Undergo the initiation process. The exact nature of the initiation may vary from tradition to tradition and even coven to coven. Wiccan founder Gerald Gardner famously instructed his students to discard rituals they didn't like and introduce ones they found worked for them. As a result, even Wiccan groups descended from Gardner -- known in Wiccan circles as Traditional or British Traditional Wicca -- have a great deal of variety in their ritual practices, with room for each High Priestess or Priest to use their creativity and intuition.
Adopt the title given to you by your new initiation. In many traditions, a first-degree initiate is referred to as "witch and priestess" or "witch and priest." This title indicates that the initiate is responsible for her own spiritual development and also capable of conducting rituals. However, this doesn't necessarily represent "priesthood" as an outsider would understand it. The true spiritual leaders of Wiccan covens are the High Priestesses and Priests.
Continue to study Wiccan ritual practices and spiritual beliefs. A High Priestess or Priest must be ready to lead the coven in magical rites.
Talk to the leaders of your coven about your interest in advancing to a higher degree of initiation. Having been through the process themselves, they will be able to explain to you what it entails. As your knowledge progresses, they may ask you to help out with coven administration or lead some rituals. In some traditions, a person leading a ritual is referred to as the High Priestess or High Priest for the duration of that ritual, as distinct from the High Priestess or Priest of the coven itself.
Develop your interpersonal and administrative skills. Just as with clergy in any other religion, Wiccan coven leaders spend much of their time organising the locations of rituals and other events, educating new initiates, mediating disputes between coven members and maintaining ties with other covens in the area and around the world. Good listening skills and force of character are needed in addition to a solid background of spiritual and magical knowledge.
Advance through the grades of initiation. As with everything about Wicca, these can vary from coven to coven, but are usually divided into three steps. In order to achieve the highest initiation as a High Priestess or High Priest, you may need either to leave your coven and found a new one -- sometimes referred to as a "daughter" coven -- or replace a High Priestess or High Priest who leaves or retires.
Although many Wiccan traditions have few rules about who can claim the title of High Priestess, resist the temptation to declare yourself one immediately. In Wiccan thought, being a priest or priestess requires you to serve, lead and educate the Wiccan community. It's not a burden to be taken on lightly or a title you should use just to impress others. It should be approached in a spirit of sincerity.