How can a Shi’i become one of Ahl al-Sunnah wa’l-Jamaa’ah?
If a person was following an ‘aqeedah that is contrary to that of Ahl al-Sunnah wa’l-Jamaa’ah, such as the belief that the Qur’aan has been altered, or that the imams are infallible and can bring benefit or cause harm, and their help is to be sought at times of hardship, or the belief that all the Sahaabah except a few apostatized, or the belief that some people will be resurrected before the Day of Judgement and that Allaah may change His mind (when new facts emerge) and that Ahl al-Sunnah are kaafirs, then the way for him to repent and become righteous is to give up these beliefs, and adopt the beliefs of Ahl al-Sunnah which is belief in the Oneness of Allaah (Tawheed) and not associating anything with Him; venerating the Sahaabah, loving them and approving of them; believing that the Qur’aan is sound and free of distortions, additions or subtractions, basing his belief on evidence from the Qur’aan and Sunnah as understood by the early generation (salaf) of this ummah.
For more information on beliefs that are contrary to that, please see the answer to question no. 60046, 44970, 101272 and 95588.
And Allaah knows best.