Urgent Advice... PLEASE (For Me)

Okay, i am fifteen and never really been in an upclose relationship. I know a quiet and shy girl but not at all very well, we spoke like twice. She seems like an extremely nice person and she acts like it (from what i see her with other people) Ive gotten to the part when i think she is getting to know me and realising me. Ive gotten quite far in my eyes but i dont want to screw it up now. Im stuck with what to say next. This is where i need some advice girls :).
Im fifteen (sixteen in two months) I want to take it slow but dont want to lose her.

Any advice, i would be thankful.

Ben