Need some answers about love

P.S.- I've never been in a relationship before. I'm 16 years old. Answer the questions number wise[Please...]. Answer in simple words.
See, I've searched the internet, I just want some answers to some questions.
But first, you all should know the case.
There's a girl, my age, and I truly love her. She's my first love. There's this another boy(lets just name him X) X. He loves her and they both, X and that girl are right now in a relationship. The girl cares about me. I told her I love her. The complication is that she had told X's sister that they both will be together, she made a promise. She cares about both of us. She said that she's breaking with both of us(X and me), as if she chooses me X will die(not literally), and if she chooses him, I'll die. Now we're in a chaos. I still love her, and I don't her to get in depression. So I told her that "I'll be okay, you just be happy and stay with X." But we all know that I didn't mean that. My love is like that I just want to see her all the time, talk to her all the time, spend time with her, want to get lost in her eyes, cry with her, simplest explanation that I just want to be with her all the time.
Now lets get out of the story, and answer some questions-
1. What happens, I mean between the two people, when they move from friends to girlfriend and boyfriend?
2. When I go to talk to her, I just get out of words, and alone I can make a whole story. So the question is how to tell her how I feel?
3. Please suggest some points, or some good lines, which might help me.
4. Should I forget her?
5. Should I be friends with her?
6. Should I start loving someone else?I mean at least try, and I'm really not sure if she'd get hurt by this.(That's a hard one for me)
7. Some questions that I can ask her.

I'll ask some more questions if I get a chance.