Al-Nawawi (may Allaah have mercy on him) said:
Repentance is essential from every sin, even if it is something between a person and Allaah and has nothing to do with the rights of another person. There are three conditions of repentance:
1- You should give up the sin
2- You should regret having done it
3- You should resolve never to go back to it.
If one of these three is missing, then your repentance is not sincere. If the sin has to do with the rights of another person, then there are four conditions: the three mentioned above and restoring the rights of that person. If it is money or property, etc, it must be returned to him; if it had to do with slandering him etc, then you should allow him to insult him in return, or ask for his forgiveness; if it had to do with backbiting about him, then you have to ask for his pardon. It is essential to repent from all sins; if a person repents from some, his repentance from the sins from which he repented is valid – according to the scholars who follow the right path – but he must still repent from the rest as well.