How do I repent to Allah Almighty and get rid of the fear after slandering a girl I do not know?

You have done well by regretting your slander of the girl and your eagerness to repent. We have already explained how to repent from slander in Fatwa no. 91426. Know that whoever repents, Allah accepts their repentance, and Allah's mercy is vast and His forgiveness encompasses all sins. Allah Almighty says: "But indeed, I am Forgiving to whoever repents, believes, and does righteousness; and then continues to be guided" [Taha: 82], and "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]. The Messenger of Allah, peace and blessings be upon him, said: "Whoever repents before the sun rises from its west, Allah will accept his repentance." Beware of sins, especially those related to the rights of people, and do many good deeds, and accompany righteous women.