Try starting off with your friends and family who perhaps know you best. Have you ever been involved in a family crisis - illness, death, emergency where you've been able to step in and take over, or help bring the matter to a satisfactory conclusion?
Has a friend been taken ill and you've taken her home and looked after her, done her shopping, called out a doctor etc?
You don't say what you're doing for employment at the moment, how old you are or whether you're a relative young person/school leaver? Have you been involved in any youth or voluntary organisations where you've been a leader or helper?