Great Shirk: worshipping beings other than Allaah (proof all over Qur'ân)
Small Shirk: Riya
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.