All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Seeking forgiveness for any sin that you have committed is done by asking Allah, The Exalted, to forgive it for you, saying, “Oh Allah, forgive my sin.”
The best wording of Istighfaar (asking Allah for forgiveness) was stated in the Hadeeth narrated by Shaddaad ibn Aws that the Messenger of Allah, sallallaahu ‘alayhi wa sallam, said: “The most superior and excellent wording of Istighfaar is to say: Allaahumma Anta Rabbi la Ilaaha illaa anta, khalaqtani wa ana ‘abduka wa ana ‘ala ‘ahdika wa-wa‘dika ma-stata‘tu, a‘oothu bika min sharri ma sana‘tu, aboo’u laka bi-ni‘matika ‘alayya wa aboo’u bi-thanbi faghfir li, fa-innahu la yaghfiru ath-thunooba illaa ant (O Allah! You are my Lord; none is worthy of worship except You. You created me, and I am Your slave; I abide by Your covenant and promise as best as I can. I take refuge with You from the evil which I have committed. I acknowledge Your favor upon me and I admit my sin; forgive me, for verily none can forgive sins except You).” [Al-Bukhaari]
Allah Knows best.