Nasuke, you're way too worked up about this. A kiss is just a kiss. Even if you kiss the girl, that doesn't mean she belongs to you, no matter how important it feels to you. Maybe she just wants to see what a kiss feels like. Unless you know how to kiss really well, she might not like her first kiss. On the other hand, if she really likes you, or if you kiss her the right way, she might really enjoy it.
I could give you some good tips, but my better judgment tells me not to, partly because you're too young, but also because you have implied that once you kiss her, you're not to going to let go of her and you're going to start "grappling" with her. Listen, if you can't control yourself, don't touch her. Tell her to get a kiss from someone else--someone who knows how to kiss, and someone who won't lose control. Or, if you really do care about her, then learn what she needs and learn how to control yourself, and then kiss her.
There's a good chance that she won't make a commitment to you, no matter how much she enjoys crying on your shoulder. There's a reason why she is with (or has been with) someone besides you. If she hasn't broken up with him already, then it's not certain whether she wants to be your girlfriend even if she leaves the other guy. She might decide that you're a good friend, but not the kind of man she wants. Be prepared for that.
Based on what you've told us, I think the best thing you can do for you AND her is to tell her how you feel about her. I have a feeling you're going to be very awkward when you try to do that, but you need experience, and the best way to get experience is to try your best and see how it works.
Good luck!
