Short answer
Sincere repentance wipes away every sin. Alongside it, the Sunnah names many deeds that erase sins: wudu, the five prayers, Jumu'ah, fasting Ramadan, an accepted Hajj, fasting the Day of Arafah, charity, saying SubhanAllahi wa bihamdihi a hundred times, and patience in illness. Rights owed to people must also be returned or forgiven.
Major sins need tawbah (repentance); good deeds expiate minor sins. A sin involving another person's right, such as taken money or backbiting, is not cleared until the right is returned or the person forgives, and missed fard prayers and fasts must still be made up.
The four schools agree on this.
Explanation
Allah says: "good deeds do away with misdeeds" (11:114). The Sunnah fills this in. The Prophet ﷺ said that when a person makes wudu well, his sins leave his body, even from under his nails. The five prayers, Jumu'ah to Jumu'ah and Ramadan to Ramadan wipe out what is between them while major sins are avoided. Whoever fasts Ramadan with faith and hoping for reward has his past sins forgiven.
More from the sahih hadith: whoever performs Hajj without obscenity or wrongdoing returns like the day his mother bore him. Fasting the Day of Arafah expiates the year before and the year after. Whoever says SubhanAllahi wa bihamdihi a hundred times in a day has his sins removed, even if they are like the foam of the sea. Charity puts out sin as water puts out fire, in a hadith in al-Tirmidhi that scholars graded sahih. Illness and hardship borne with patience also remove sins.
For major sins, the scholars require tawbah: stop the sin, regret it, and resolve not to return. Then three things remain. If you owe someone a right, return it or ask their pardon. If you missed fard prayers or fasts, make them up; see the qada calculator. And follow the sin with good: "fear Allah wherever you are, and follow a bad deed with a good one, it will wipe it out".
Many find it helps to pray Salat al-Tawbah and say Sayyidul Istighfar. See also how to repent, does sickness wipe away sins? and the Day of Arafah.
The evidence
Quran, Surah Hud 11:114
وَأَقِمِ ٱلصَّلَوٰةَ طَرَفَىِ ٱلنَّهَارِ وَزُلَفًۭا مِّنَ ٱلَّيْلِ ۚ إِنَّ ٱلْحَسَنَٰتِ يُذْهِبْنَ ٱلسَّيِّـَٔاتِ ۚ ذَٰلِكَ ذِكْرَىٰ لِلذَّٰكِرِينَ ﴿١١٤﴾
And establish prayer at the two ends of the day and at the approach of the night. Indeed, good deeds do away with misdeeds. That is a reminder for those who remember.
The general rule that good deeds erase sins.
Quran, Surah Al-Furqaan 25:70
إِلَّا مَن تَابَ وَءَامَنَ وَعَمِلَ عَمَلًۭا صَٰلِحًۭا فَأُو۟لَٰٓئِكَ يُبَدِّلُ ٱللَّهُ سَيِّـَٔاتِهِمْ حَسَنَٰتٍۢ ۗ وَكَانَ ٱللَّهُ غَفُورًۭا رَّحِيمًۭا ﴿٧٠﴾
Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.
Shows repentance turns even grave sins into good.
Sahih Muslim 245
Sahih... مَنْ تَوَضَّأَ فَأَحْسَنَ الْوُضُوءَ خَرَجَتْ خَطَايَاهُ مِنْ جَسَدِهِ حَتَّى تَخْرُجَ مِنْ تَحْتِ أَظْفَارِهِ ...
"Whoever makes wudu and does it well, his sins leave his body, even from under his nails."
Shows wudu removes sins.
Sahih al-Bukhari 1521
Sahih... مَنْ حَجَّ لِلَّهِ فَلَمْ يَرْفُثْ وَلَمْ يَفْسُقْ رَجَعَ كَيَوْمِ وَلَدَتْهُ أُمُّهُ ...
"Whoever performs Hajj for Allah without obscenity or wrongdoing returns like the day his mother bore him."
Shows an accepted Hajj wipes away sins.
Sahih al-Bukhari 6405
Sahih... حُطَّتْ خَطَايَاهُ، وَإِنْ كَانَتْ مِثْلَ زَبَدِ الْبَحْرِ ...
"His sins are removed, even if they are like the foam of the sea."
Reward for saying SubhanAllahi wa bihamdihi a hundred times.
Jami' at-Tirmidhi 1987
Grading to be checked... اتَّقِ اللَّهَ حَيْثُمَا كُنْتَ وَأَتْبِعِ السَّيِّئَةَ الْحَسَنَةَ تَمْحُهَا ...
"Fear Allah wherever you are, and follow a bad deed with a good one, it will wipe it out."
Shows good deeds after a sin erase it.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Book of Hajj, on what Hajj expiates. Explains that Hajj does not remove the rights of people or the duty to make up missed obligations.
- Riyad al-Salihin (al-Nawawi), Chapter on repentance. Gives the conditions of repentance and the extra condition for rights owed to people.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.