What should I do to repent after guaranteeing a friend's usurious loan from the bank, when I did not know that was forbidden?

You must sincerely repent. This means immediately ceasing the sin, regretting what has passed, resolving not to return to it, and seeking forgiveness. For whoever repents, Allah accepts their repentance. Allah Almighty says: "O you who have believed, repent to Allah with sincere repentance. Perhaps your Lord will remove from you your misdeeds and admit you into Gardens beneath which rivers flow." And Allah Almighty says: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills." And the Prophet, peace and blessings be upon him, said: "All the children of Adam are sinners, and the best of sinners are those who repent."