Slander is forbidden to all Muslims, and he who slanders has sinned in every instance. Slander is when you mention something about a person which is not a fault in Allah's eyes, or when you censure what the people of knowledge praise.
As for discussing someone who is not present with regard to something which Allah censures, when that person is guilty of it, then this does not amount to slander, even if he dislikes it when he hears it; and you are free of any slight of that person. This is in order to make the truth clear from the false by the clarification of Allah and His Messenger. However, this has a precondition, in that the person who says it must seek only to clarify what is true and false, in the faith of Allah. If he means to deprecate the person of whom he talks without meaning to make things clear, then he is taken to task for his corrupt goal, even if he is correct in what he says.
If you really slander someone, then seek pardon from that person. If you do not go that far nor reach that point, then ask Allah's forgiveness for that. Slander eats up good deeds as fire eats up wood. As Allah revealed to Moses, 'The slanderer will be the last to enter the Garden, if he repents. If he does not repent, then he will be the first to enter the Fire.' As He said,
Does one of you like to eat the flesh of his dead brother? You would hate it. (49:12)
The various aspects of slander occur when you mention a fault in someone's character, intellect, action, behaviour, belief, ignorance and so on.
The origin of slander may be one of ten types: venting one's anger, pleasing other people, suspicion, believing a report without investigating it, having a bad opinion, envy, mockery, astonishment at some action in another which one does not comprehend, dissatisfaction or impatience with others, and embellishment of oneself at the expense of others.
If you seek Islam, then remember the Creator and not the created; then the circumstances of slander will be a lesson for you, and a wrong action will be replaced by a reward.