Who can nominate a person to become a saint?

The local parish, diocese or religious order of the possible candidate is responsible for preparing and pursuing the case for sainthood. They submit their evidence and information to the Sacred Congregation for the Causes of Saints in the Vatican. They will review the information provided and make a decision to open a 'Cause' for the person. Additional information will be required as well as proof that the person interceded on someone's behalf and brought about a miracle or miracles - proof that the person is, indeed, in heaven. Once the final stage is reached, a hearing of sorts is held by members of the Congregation and they may choose to propose the candidate to the pope for canonization. The pope makes the final decision.