Okay, so I've recently gained guardianship of a 12 year old girl, and since I've never raised a child (I'm only 23) I'm finding it difficult to know exactly where to draw lines as far as raising her. It isn't a problem of getting to know her; I was unofficially taking care of her since she was ten, so we are very comfortable with each other and I care for her very much. The problem is that she has never lived with me or had to follow my rules, and now I'm finding it difficult knowing what rules to set in place for her, and how seriously to take it when she doesn't follow them. She is a very good girl with a sweet heart, and she isn't really causing trouble aside from her not really taking my rules seriously: such as my curfews, or needing to call me before she leaves a friend's house; simple things like that. I'm wondering if it's perhaps because she's never been raised by a guy before, so she doesn't really see me as an authority figure for that reason, or if it's just because we were more like friends than parent/child for most of our relationship. I know I need to be firm, but it's just that she's never really had friends before this point in her life, so I'm finding it difficult to be mad at her for getting carried away with her new relationships; at the same time, I don't want to let her down in the long run by not being strict enough to teach her discipline.
I suppose I'm just looking for general advice, and how I should go about sitting her down and telling her that I'm not just her friend anymore, but (kind of?) a parent. What approach should I take? Should it be a stern talking to (which I've never really done with her before) or just a casual conversation while out shopping or something?
I know I shouldn't feel so overwhelmed at such a small problem, because like I said: she's a great kid overall. I'm just new at this, I guess...
Any advice would be appreciated.