The Shahada (Arabic: "the testimony") is an Islamic oath and creed, and one of the Five Pillars of Islam and part of the Adhan. It reads: "I bear witness that there is no deity but God, and I bear witness that Muhammad is the Messenger of God."
The Shahada declares belief in the process tawhid (Arabic: "oneness") of God and the acceptance of Muhammad as God's messenger. Some Shia Muslims also include a statement of belief in the wilayat (Arabic: "state") of Ali, but they don't consider it as an obligatory part for converting to Islam. So a single honest recitation of the Shahada is all that is required for a person to become a Muslim according to most traditional schools.