Will a person be held accountable for sins committed before knowing their prohibition, such as watching pornography and masturbating? And will their sincere repentance from these be accepted? What are the ways to get rid of these sins?

Pornographic films and masturbation are forbidden acts, but your feeling of remorse and your seeking repentance are a good sign. Allah accepts the repentance of His servant as long as the soul has not reached the throat (i.e., before death), and whoever repents with sincere repentance will not be punished by Allah. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" [Az-Zumar:53]. And prayer restrains from indecency and evil. As for ignorance of the ruling, if you were excused for it, then you are not sinful.