>>1 In the sense of taking some form of romantic action with a friend, be aware of a few things first.
-Is this someone you really care about romantically, or is your mind forcing you to think about her? Concern and infatuation are two different things.
-Not to be insulting, but the way you write makes me think you need to check your hormones. Don't go and ruin a friendship simply because you're in the angstyloveteen stage of your life.
-Does she believe the same things you do? I used to disregard this as a non-issue when I was younger, but you seriously need to consider it.
-Has she given you any inclination that she ever thinks about you the same way you're thinking about her? The demon in this one is infamously named "Friendzone". If it doesn't seem like she does, don't tell her outright. Stir up a little chaos and lead her to consider it indirectly. I guarantee you that it'll be better for her to think she likes you on her own than it will for her to think she likes you with your encouragement.
If all these things are clear, only then can you have the greenlight.