Du'a for Repentance
Whenever a person commits any sin, he/she should immediately offer repentance to Allah, saying the following dua.

اللّهُمَّ إِنِّي أتُوْبُ إِلَيْكَ مِنْهَا لاَ أَرْجِعُ إِلَيْهَا أَبَداً

O Allah, I repent before You for all my sins and I promise never to return to the same (again).