How do i get her to say yes?

I'm 16 going on 17 in August, but i have a very special best friend and we have been best friends since 5th grade. now we are juinors, anyways we have dated before but didnt work out and today she had a very bad break up with another boy but she seems to be ok with it but ik shes not and im trying to help her through it. anyways ik its wrong to ask her out after a break up so maybe i will wait for awhild even tho she will say no but i just dont know how to tell her how i feel about her because last time i told her she got mad at me idk i guess thats how girls are but i think shes truly the one but idk how to tell her that. idk maybe im stupid or something maybe i dont have a true love god only knows but to me when i look at her i dream about the future of me and her i... i just dont know how to tell her and when to tell her and idk how to get her to date me i need help