What should a committed young man do when he falls into some sins like masturbation and looking at obscene pictures, and then regrets his actions?

You must offer sincere repentance, increase your acts of seeking forgiveness (istighfar) and righteous deeds, and turn to Allah for protection from Satan. Recite the Quran frequently and ponder its meanings, and be mindful that Allah is aware of you. Adhere to righteous companionship and distance yourself from evil companions. Abandon and sever ties with the causes that incite disobedience, for this is the first gate of repentance and proof of the sincerity of the one who repents.