The Charge of the Goddess is a remarkable Wiccan work, originally written by Doreen Valiente. She drew its language from many different sources, including some ancient invocations and late 18th century work. This article, and this one, both give an excellent explanation of the history of the Charge of the Goddess.
The Charge is probably most well known for being used in full moon rituals when the trance rite called Drawing Down the Moon takes place. In this rite, the practitioner invokes the Goddess directly into themselves. The Goddess may speak through them. Since I am still learning about this practice myself, I hesitate to give any information. The only thing I would say is that some material acts as though this is a purely coven-based activity or something only a High Priestess can engage in. I don’t believe this is the case. We are all priests and priestess of the God and the Goddess by virtue of being Wiccan, and we have the divine right to engage in this ourselves. What you will experience, however, varies widely depending on experience, atmosphere, expectations, and other similar factors, so don’t give up if you don’t have an earth-shattering experience the first time.
Here is a good article on Drawing Down the Moon. You can also read about it in Raymond Buckland’s book “Wicca for One” and Phyllis Curott’s book “Witch Crafting”- which I recommend in that they are both targeted towards solitaries.
There are many variations of the Charge of the Goddess. The one I will post here is from my Book of Shadows, but I don’t remember where I found it originally.
Listen to the words of the Great Mother, Who of old was called Artemis, Astarte, Dione, Melusine, Aphrodite, Cerridwen, Diana, Arionrhod, Brigid, and by many other names: Whenever you have need of anything, once a month, and better it be when the moon is full, you shall assemble in some secret place and adore the spirit of Me Who is Queen of all the Wise.
You shall be free from slavery, and as a sign that you be free you shall be naked in your rites. Sing, feast, dance, make music and love, all in My Presence, for Mine is the ecstasy of the spirit and Mine also is joy on earth. For My law is love is unto all beings. Mine is the secret that opens the door of youth, and Mine is the cup of wine of life that is the cauldron of Cerridwen, that is the holy grail of immortality.
I give the knowledge of the spirit eternal, and beyond death I give peace and freedom and reunion with those that have gone before. Nor do I demand aught of sacrifice, for behold, I am the Mother of all things and My love is poured out upon the earth.
Hear the words of the Star Goddess, the dust of Whose feet are the hosts of Heaven, whose body encircles the universe: Who am I the beauty of the green earth and the white moon among the stars and the mysteries of the waters, I call upon your soul to arise and come unto me. For I am the soul of nature that gives life to the universe.
From Me all things proceed and unto Me they must return. Let My worship be in the heart that rejoices, for behold, all acts of love and pleasure are My rituals.
Let there be beauty and strength, power and compassion, honor and humility, mirth and reverence within you. And you who seek to know Me, know that the seeking and yearning will avail you not, unless you know the Mystery: for if that which you seek, you find not within yourself, you will never find it without. For behold, I have been with you from the beginning, and I am That which is attained at the end of desire.“