She is 17, which means she really isn't a "little"girl. She is old enough to make her own choices,And no it doesn't mean she'll make the right ones.IF you tell her dad, that'll look really bad on you, if you think she is in any kind of danger then yeah go for it, help her out. But on the other hand she could be a little slut, and just be doing it because that's how she is.

As for you. Live life, be happy, Smoke some weed, And enjoy your teen years. Date, and don't get serious just yet. U'll be fine.

Whatever it is, Good luck with life, and Whatever you chose to do make sure you smoke a fatty after your big"choice"

Personally, I'd say fuck it, and leave her alone(unless you do think she is in some sort of danger). Smoke a bowl and get back into the dating scene