I have a boyfriend but I kissed someone else once and someone knows. What do I do?

go and tell your boyfriend but say it happens just once and if he don't liked it or angry with you, you are never going do this again because if he do this to you and cheat you and someone else tell you this you also not feel good. Respect his feelings and tell him say you are sorry and will do things that's you can also digest if someone else do with you in who is with you in relation. Don't let anyone ruined your inner peace and respect those who are important for you. God bless