Ask the league how they like it written on the game card. Usually there is a comment section where you can note something like "Goal scored in 30th minute by 13 of own team." I usually just leave the player # field of the card blank or put an asterisk. There is no universal way for reporting an own goal, every league has a different method if that information is even requested (anything below adult amateur or youth SPL/ODP don't usually care who scores a goal).