A person has to develop faith in his heart and be convinced that Islam is the truth and then he will himself revert to Islam. Reversion to Islam is done by proclaiming the Shahadah, testimony of faith, by saying:
أشهد أن لا إله إلا الله وأشهد أن محمداً رسول الله
I bear witness that there is none worthy of worship except Allah and
I bear witness that Muhammad is the messenger of Allah