The beginning of Islam is not separate from the beginning of Shi'ism; it started with the Prophet of Islam, Muhammad al-Mustafa (s.a.w.), and has been preserved in its origin form by the Imams of Ahlul Bayt (a.s.).
The Prophet presented the message of Islam in the da'wat dhu 'l-'ashira and also introduced 'Ali as "my brother, my successor, and my caliph." And then just a few months before his death, in the biggest gathering of his life, at Ghadir Khumm, the Prophet clearly appointed 'Ali as the mawla (the master, the leader) of the ummah. In between the da'wat and Ghadir Khumm, the Prophet introduced 'Ali to the people in various occasions. The appointment of 'Ali for imamat and khilafat was explicit and clear.
'Ali and the Imams from among his descendants are the Ahlul Bayt in the Qur'anic term. Loving and honouring them is an Islamic duty of all Muslims. The Ahlul Bayt have been vested by Allah with the wilayat in the broadest sense of the word, and that also includes the 'ilmu 'l-ghayb.