The major sins are those acts which have been forbidden by Allah in the Quran and by His Messenger (SAW) in the Sunnah (practise of the Prophet), and which have been made clear by the actions of of the first righteous generation of Muslims, the Companions of the Prophet (SAW).
If you avoid the great sins which you are forbidden to do, We shall remit from you your (small) sins, and admit you to
a Noble Entrance (i.e. Paradise). [Al-Quran - 4:31]
Please make sincere repentance to Allah before as Ali (ra) said, "Today is deed without reckoning and tomorrow is reckoning without deeds". Sincere repentance has four conditions :
* Feeling bad for the sin
* Firm commitment in intention not to repeat sin (whether it happens again is not a condition if one tried his best)
* Make repentance to Allah by Du'a and asking or better crying for forgiveness
* If some person has been wronged because of this sin then one needs to make up to this person