Asalam o.Alaikum!!!What is the repentance from accusing someone of zina, theft or any other thing?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

The due expiation for such an act is sincere repentance. The conditions of the sincere repentance have been highlighted in Fatwa 86527. Since such an offense is associated with another person's right, i.e. the wronged person, the seeking the forgiveness of the person concerned is obligatory because these are the rights of human beings. The right here is moral; you should seek his pardon for whatever reaches him of your slander. However, you should not inform him of what did not reach him; it is enough for you to supplicate Allaah to forgive him and bless him with endless bounties. For further information, please, refer to Fatwa 81748.

In fact, some scholars stipulated another condition for the validity of the slanderer's repentance. They argued that the slanderer should admit his untruthfulness concerning the accusations he cast against the wronged person. Ibn Qudaamah wrote, “The apparent meaning of Imaam Ahmad’s and Al-Khiraqi’s statements is that the repentance of the slanderer is to declare that he was lying. So, he should say, "I was lying when I said that." This is the opinion stated by Ash-Shaafi‘i, and the one adopted by Al-Istakhri, the Shaafi'i scholar. Ibn ‘Abd Al-Barr said: “Those who adopt this opinion include Sa‘eed ibn Al-Musayyib, ‘Ataa', Taawoos, Ash-Sha‘bi, Is-haaq, Abu ‘Ubayd and Abu Thawr. This is based on the report narrated on the authority of Az-Zuhri from Sa‘eed ibn Al-Musayyib that ‘Umar said that the Prophet, sallallaahu ‘alayhi wa sallam, commented on the verse that reads (what means): {Except for those who repent thereafter and reform, for indeed, Allaah is Forgiving and Merciful }[Quran 24:5] saying, “His repentance (i.e., the one who accused someone of Zina) is to say that he was lying…” [Al-Mughni]