What am I to do? (Relationship Issue!)?

Ok, I'm in love with a girl. She is fun, active, hilarious, smart, her judgment is impeccable and she is by far the most beautiful girl I have ever seen, lets call her M for secrecy's sake. Now, the issue: I don't know what I am going to do about her. We have been great friends for four years (we are both sixteen), and it started after two years. I thought it was just a teenage thing that would p, so I began a short relationship with a pretty nice girl. But, it didn't last because the only girl I could see myself with was M. She has never had a boyfriend before because she is rather of wary of guys given their stereotypical "wants". I love her, I really do, but I don't know how to approach her about deepening our relationship without possibly destroying what it is we have now. Also, I think she already knows to some extent...