Rulings on sins committed against other people before being guided

Allaah has enjoined repentance
upon His slaves, as He says (interpretation of the meanings): 

“And all of you
beg Allaah to forgive you all, O believers, that you may be successful”

[al-Noor 24:31] 

“O you who believe!
Turn to Allaah with sincere repentance!”

[al-Tahreem 66:8] 

“And verily, I
am indeed forgiving to him who repents, believes (in My Oneness, and
associates none in worship with Me) and does righteous good deeds, and
then remains constant in doing them (till his death)”

[Ta-Ha 20:82] 

And the Prophet

(peace and blessings of Allaah be upon him) said: “The one who repents
from sin is like one who has never sinned.” 

Whoever commits any sin
must repent, regret what he has done, give it up, keep away from it
and resolve not to go back to it. Allaah accepts the repentance of those
who repent. 

When a person sincerely
repents by regretting what he has done and resolving not to repeat it,
and he gives it up out of respect and fear of Allaah, then Allaah accepts
his repentance and wipes out his past sins by His grace and kindness. 

But if the sin involved
wrongdoing against someone else, then the sinner has to do what he is
obliged to do by repenting from what has happened, regretting it, giving
it up and resolving not to do it again. He must also restore the rights
of the one whom he wronged, or seek his forgiveness , such as saying,
Please forgive me,” etc. Or he should give him his dues, because of
the hadeeth quoted in the question, or other ahaadeeth and aayahs. The
Messenger

(peace and blessings of Allaah be upon him) said: “Whoever has done
some wrong to his brother, let him seek his forgiveness today before
there is no dinar or dirham, when if he has some righteous deeds (to
his credit), some of his hasanaat (good deeds) will be taken in proportion
to his wrongdoing, and if he has no hasanaat some of the sayi’aat (bad
deeds) of the one whom he wronged will be taken and added to his burden.”
(Narrated by al-Bukhaari in his Saheeh). So the believer should
strive to put things right and restore his brother’s rights. So he should
either give him his dues or ask him for his forgiveness. If it has to
do with his honour then he has to ask for his forgiveness if he can.
If he cannot, or he fears the consequences of telling him or that telling
him about what he said will cause more trouble, then he should pray
for forgiveness for him and make du’aa’ for him, and mention his good
points that he knows of , to compensate for saying bad things, aiming
to wash away something bad with something good. So he should speak of
his good points that he knows of and spread good words about him to
counteract the bad things which he said before, and he should pray for
forgiveness for him and make du’aa’ for him. In this way the problem
will be solved.