How do I ask my older friend to be sort of a mentor?

I am a Christian. I kind of have a problem with putting my feelings away and I don't talk about my problems. I like to help others with their problems and not deal with mine. I bottle everything up and then when I am around people they don't have any idea about anything. I really want to work on that but I don't really have anyone to talk to about it. I am in High School and I have a friend who is in College and I feel like I can trust her. She is going to be a leader at the Church camp we are going to next week. I want to maybe talk to her about mentoring me? (I'm not really sure that is the right word for it.) I want to see if she would maybe become closer friends with me and be some one who I could talk to and get advice from. But the problem is, I don't know how to ask her. How do I? How do I talk about this? Do you have any advice in this at all? Any advice is much appreciated.