Like most stories, this one takes place in a world identical to ours, except in this one, on their 18th birthday people's signs awaken. When a person's sign awakens they gain a variation of an elemental power based on their zodiac sign.
All the signs base element.
Fire signs: Aires, Leo, and Sagittarius.
Earth signs: Tarsus, Virgo, and Capricorn.
Air signs: Gemini, Libra, and Aquarius.
Water signs: Cancer, Scorpio, and Pisces.
Ask any questions in the comments, and I'll try to answer them here!