I'm biual and falling for my straight friend? help!?

I'm really close with my best friend. she knows i'm bi, and she doesn't have a problem with that. it's just that lately, i'm starting to really like her, i always think about her, she is just incredible and i love being around her. to put it in simpler words. i like her more than just my best friend. i want to have something more than that. but i know that will never happen because she is straight. i still want to tell her how i feel, but i'm not sure how to tell her, or when, and what she would think. i don't want her to treat me differently in some way if i tell her that i really like her and that i want to be more than best friends. should i wait to tell her? i'm a 15 year old teenager.