By His Grace and Mercy, Allah has made embracing Islam a cause to erase the sins that were committed before it. When a disbeliever becomes Muslim, Allah forgives all that he did when he was a non-Muslim, and he becomes cleansed of sin.

1- It is not essential to take shahadah (say the testimony of faith) before witnesses. Islam is a matter that is between a person and his Lord. 2- No marriage contract is valid without a wali (guardian) for the wife, but no non-Muslim can act as a wali for a Muslim woman. For more, see the detailed answer.